The KC64EXP is a multislot cartridge that allows to connect 2 cartridges to the expansion port.
You can enable cartridge in slot 1 or the one in slot 2 or even both of them together if they are compatible.
You can use the Swap I/O Lines feature to try to make work soma not compatible cartridges.
The data lines are fully buffered to “clean” data passing through in case of “noisy” c128.
Feel free to ask any info at: firstname.lastname@example.org