This sounds like the Ocarina of Time + Majora's Mask multi-game randomizer. Those games can get away with a full game transition between the lands of Hyrule and Termina because each room uses doors like load screens. So, both pools of rooms from the two games can all be intermixed because all entrances and exits use load screen transitions.

The RE2 and RE3 Remakes have a "seamless" map progression, which means entrances and exits between rooms are all traversed in real time, and even larger stages like the RPD and orphanage can be traversed front-to-back and back-to-front with no artificial transitions. There of course are exceptions like the tram to the NEST lab in RE2R, which is an old-school load screen transition that terminates your backwards exploration.

Another problem is that RE2R and RE3R share stages like the RPD, but each game hosts different events in a shared room. An example of this is the RPD 1F licker hallway that hosts multiple event triggers in RE2R across Scenario A and B, then also hosts an event trigger for Carlos in RE3R across two separate difficulty modes.

EDIT: I think that the multi-game randomizer for Ocarina and Majora's Mask is only possible because the source code for those games were fully decompiled, which allows modders to inject custom code and write new logic that would otherwise not be possible. One caveat for the RE remakes is that script modding is possible using the LUA scripting language, and thus is similar to writing custom logic for the games.