Back in the old days, radios on the two systems couldn't (or wouldn't) be programmed to talk to each other. As a result SDPD and county law enforcement dispatchers would create patches between talkgroups on the two systems. Some pursuit on SDPD Southern Dispatch 1 could be patched to LE South Command for example. When patching an RCS talkgroup incident to the city system, traffic would always be patched to one SDPD talkgroup creatively named "PATCH", and officers would have to "switch to patch" to hear the RCS traffic.
Nowadays the city system talkgroups and RCS talkgroups are just two zones in the same radio, so the officer can easily switch over if they wish. The downside to this is that the radio will only priority scan talkgroups from the trunking system belonging to the talkgroup it's currently on, which a lot of radio users don't realize. So if I'm an SDPD officer on Mid City Dispatch 1 and I switch to LE South Tac 2, my radio will stop scanning any of the SDPD channels and I won't hear my dispatcher calling me until I switch back to a talkgroup belonging to the city system (or keep a second radio nearby on a city system talkgroup). It's worth noting newer APX radios do support a "multi-system scan" option that fixes this limitation, but I'm not sure if it's programmed or not.
Training and dispatch center procedures haven't necessarily caught up to the fact that both systems are programmed in the radios though either, so city and county dispatchers will still work together to create patches between the city system and RCS, largely out of convenience for the officers who may already have their hands full (imagine fumbling with changing zones on the radio in the middle of a high speed chase). SDPD "PATCH" still gets used, but if they patch SDPD "PATCH" to RCS "LE S CMD", it's a toss up on whether the officers switch to PATCH or just go directly to the RCS talkgroup since they have both programmed. The radio programming in the LE helicopters has always been excellent, so ABLE/ASTREA will almost always affiliate to the original talkgroup an incident is occurring on and not a patch, and can talk to conventional users like CHP on VHF low band, Cal Fire on VHF, etc.
Long story short, some really smart people worked on communications interoperability in San Diego/Imperial counties over the last several decades (some who used to participate on this forum even), and in my opinion it's very well thought out.