I did my butt joints by glassing only the inside, avoiding the sanding altogether. Didn't have any troubles (this was an Osprey Std.) Others here have done the same, tho I don't remember any discussion on a double. There has been some discussion with Pygmy on this general issue, and they seem to expect that the deck glass overlap is going to be sanded off anyway in finishing.

My .02 - you'll be fine with the deck tape sanded off. I'm a bit less confident advising sanding the hull tape, but it'd probably be OK.