Max GBA ROM space is 32 megabytes/256Mbit, at least until you try bankswitching ( https://mgba.io/2015/10/20/dumping-the-undumped/
), and full space is available to and always fully in memory for each and every game on the system bar those from the link. Flash carts had this in some iterations but let us not go there, for some forum searcher then the exception from homebrew tended to be pogoshell.
Very few people will build GBA multiroms as flash carts were so prevalent and geared towards multiple games from menus. In commercial games things when multipack games appeared tended to be compiled for it (ASM was not unknown to commercial GBA coders but C was very much the standard for most things) and just branched to/soft reset to, the most interesting stuff you will see will be for phantasy star collection and possibly Bauldur's gate which did things differently and troubled cheat device users as a consequence.http://doc.kodewerx.org/hacking_gba.html#nonstandard
To do a multi game you will either have to find a way to do virtual pages within a game (not going to happen, flash carts did this with hardware if they did it at all*), remap everything in the game which is as tedious as it sounds or create some kind of abstraction layer for reads (as it is all in memory you might map something to 09?????? and then any reads to the original 08?????? region should not be hard to pull off). Pointer maths, calculated pointers and similar concepts are common enough on the GBA as the processor is kind of geared for it in some cases (it does well at shifting for one) so if you were thinking some kind of static analysis then consider it carefully, it is however not a required thing so you may dodge that bullet. Owing to the tools made for it (a guy named Atrius did some fine work on it, including tools that scanned for files and had a lot of baked in info about the pointers) you might be able to get a lot done on the static analysis side of things however.
Likewise that extra space being available means you will definitely not be able to combine it with some of the nice hacks like the voice acting experiment.
*older NOR based stuff will, later RAM/dump section based things most likely will not or will have a lot of stuff in the middle of it all to handle saves.
Personally I think it is a silly idea. By all means have something to share any kind of save data you want but a fusion hack is a lot of work for no real gain. The "best" way for anything close for me would be a custom build of an emulator that holds the other in reserve, hack in an interrupt at a key point, sort any save/character info and then soft reset or branch into it a la how a lot of flash carts worked. Not really what people seem to go in for around here and still fairly pointless from where I sit but to each their own.