How big is the span and do you know what the header situation is?

For the purpose of a slider, it measures 71"W x 35"H. I'm not exactly sure what a header is (sorry), but if it's the peice of wood across the top of the window, it's approximately 7" in width, 2" thick and spans the length of the opening. Hope this helps.

Yes and no.

You will need to either look into the span tables or consult with an engineer to see if that wood will accommodate that span in your case.

You don't want to start removing structure and create a larger issue.

I know that you mention in your post that it is "solid", but have you verified that? Most mullions (especially in that size and with a continuous header) are either hollow chambers or simply the two window buckframes strapped to/abutting each other. In either of those cases, take them out no worries. If there are actually studs between the units, then you will want to follow WoW's advice and look into that further. If that is a doubled 2x8 over a 6' opening I think it should suffice, but I'd double check.