For proper Super Game Boy support we need support for multiple simultaneous emulated machines. The SGB cartridge contains an entire Game Boy system, with its own CPU, RAM, and a modified PPU that DMAs its output into the SNES' VRAM instead of driving an LCD. The GB side and SNES side are quite separate and have very limited bandwidth to communicate with each other (that's why SGB games need to pause for so long to upload a border on real hardware)
