What I see is a initial "set up" story where they encounter each other and the rules are established. Subsequent stories are pretty much stand-alone, same universe.
To keep the magic wand of technology from solving everything, the setting is pre-2000. (Make that 2001, so allow a Y2K plot or two). This means that
- Google exists ONLY as a search engine, no g-mail. Yahoo is king of the internet. AOL and AOL keywords are the internet portal. IRRC, BBS, alt. groups, and messenger are state of the art.
- Only larger companies had hardwired internet. Many homes had a second phone line for dial-up access.
- Cells phones exist, but are analog. The most advanced have on-board solitaire. Digital cameras are separate point and shoot units with memory cards.
- GPS is used by ships and land surveyors. Cars are tracked by attaching radio transmitters and following them with direction finding receivers.
- Accounting is done with paper, tax forms are mailed in with W2s attached. You carried check cashing cards for grocery stores.
- Credit card authorization is by phone and floor limits. Oil companies and Department stores still run in-house programs.
- Credit scores don't exist. Transunion, Experian, and Equifax keep data submitted by banks and merchants. User Profiling is just some executive's pipe dream.
- You could still go to the airport and walk on to a plane without a reservation, ID, or going though a metal detector.
One "spy" is a private investigator specializing in skip tracing -- finding people -- and occasionally will help someone "get lost". Also does deep background checks, e.g. locate/interview people from the subject's past. Has contacts galore for "private" records. Owns or works for the small business SkipTrace, Inc.
The other "spy" is a corporate security type, one of three working for the large XYZ company. One does plant (physical) security, another does internal controls and EDP (fast evolving into IT) security, and our spy does employee watching and corporate (anti-)espionage. The co-workers have access to lots of resources.
I'm still figuring out how they meet each other. Crossing investigations, perhaps subcontracting background check.
They stabilize in a fuck buddies sharing an address relationship, maybe a weak polyamory. They won't commit to each other, but are highly protective of each other and watch each other's back. Perhaps they "love" each other but will never openly admit it. They recognize being married to each other wouldn't work. Probably neither is the marrying kind.
The first story might be
An XYZ employee is being seduced to expose corporate secrets. The seducer works for InfoMax, a security firm specializing in corporate espionage and hired to find XYZ's secret project. The seducer just finished an InfoMax job against ABC, and when ABC found out they got burned, they hired SkipTrace to find the seducer and connections.
When XYZ security discovers their employee being seduced, they sub out to SkipTrace to trace the seducer. Or XYZ the security person, i.e. spy #2, stumbles across the InfoMax person, i.e. spy #1, following the same trail.
The two spies conceive running a misinformation operation on the employee/InfoMax pair having the affair. This expands to a sting operation against InfoMax and/or who hired them for the XYZ attack.
During the enterprise, our spies have a strong sexual attraction to each other and each investigates the other kinda of like due diligence. They discover each having secret affairs with other people, or perhaps just quiet affairs where multiple people have to be kept in the dark about each other.
As part of the sting the spies present themselves as in a relationship, and it works well enough that it becomes more than acting. They finally have sex as part of their 'cover', and they keep on fucking.
Finally one of the spies turns the conversation serious, and after much hedging around what they know from investigating each other, they settle on 'keeping their cover going' after the case ends. It's much like a game to both of them, so they get inspired and set some guidelines where spying on each other is OK.
Both spies are really good hearted people, and over balance the ruthlessness required in their professional lives with kindness in the personal lives. They also thrive on the NRE (New Relationship Energy), so want ongoing turnover in their affairs. And they want to keep their "special" relationship with each other.
Time comes one wants to break off an affair, and enlists other's help in a MI operation to send the secondary on with their life in a good way. The secondary will always look back on the spy with affection, like for one's first lover, and suffers no (minimal) heartbreak.
This all jells into the system of notes written to each other in advance of need to know and lets unknown timings and actions drive each new story.
Spy has down time, wonders what other spy is doing, investigates, decides affair may be ready to end so consults the note file for instructions. Could just be other spy's behavior suggests a check of the note file is needed.Followed by
A rumor's on the street that the other spy is being setup.
One spy asks the other to watch their back during some risky encounter.
One spy has a task and suggests a joint operation with the other.
A common friend of the spies gets in trouble and they choose to intervene.
some operation(s), hopefully with a few interesting twists and turns.Gratuitous Sex
Could be inserted lots of places.
Maybe the spies are surprised at first professional meeting by already knowing each other from a sex club?
Plenty of room in spies checking each other's affairs...
Most obvious is the plot setup -- watching the seduction which is the base for blackmail/theft/embezzlement that the spies solve. q.v. first story thoughts.
But the really best method is making it integral to the sting or MI operation. It could draw victims into the sting. It could feature in the unraveling, victims outed on public display, or blackmail to stop a takeover, etc.
Or it could be overall plot, one spy is being seduced by a DOM that subjugates the conquests so the spy may need rescuing.