Merge 2 Domain from seperate forests

Hello Guys,
I have a problem and need to canvass for opinion on how to procede.

I have two domains which are in seperate forests that need to be joined. curently have mydomain.com & my2nddomainelsewhere.com and need to end up with mydomain.com & us.mydomain.com within the same forest.

What way is best to go about making this change. I have looked into rendom however once renamed it states you cannot change the root domain of the forest which is what I need to do I believe.

Any suggestions would be greatly appreciated.