EA Modify External Variables of Another EA?

No, one EA cannot directly alter "extern" or "input" variables of another EA.

However, you can share "Terminal Global Variables" between EA's running on the same "Client Terminal" process, so that if one EA alters the values of "Terminal Global Variables", the other EA can see it, provided that other EA regularly checks their contents.

Alternatively, using system DLL's you can use inter-process communication to achieve something similar, but this is more complicated and requires programming knowledge, probably beyond what you are seeking.