i asume we can migrate just that app
Why do you assume this? How does the app work? How does it authenticate? How much data does it have?, etc, etc.
i keep missing the quotes .. if AD is merged, exchange merged, RDP and the only outstanding from company B is the APP , instead of bringing the entire infrastructure just to run the app, i can migrate the app,
Why are you merging twice though? or more like.. merging once and migrating once?
Merging company B into company A, then migrating company A into company C? Why not skip all that and just create C and migrate everything directly into it?
That's how I feel. Start this now, move fast. Might be done in two weeks with six weeks to sit back and relax.
that would be fine if no one was going to work for the next two months or no changes where going to be made the main data migration still need to be done over a week end and out of hrs.