Both Guitars At Once?
 

How do I get both parts of a harmony to play and be displayed on separate staffs? And do I need to write the entire song for both guitars or can I get away with just writing the 2nd guitar when it differs from the 1st?

Assuming you're talking about PowerTab an dnot Guitar Pro:

Yes you can do this, you just need to press the Attach Staff button. Make sure you set up and assign a 2nd guitar to it.

For parts that both guitars share, just make sure you insert both guitars to the same staff.

I found the answer since posting this thread but thanks anyway!