You can access both GC lounges. Your key card should already be programmed to access the club level at both towers or you can ask the staff who check in to make sure this is the case.

I've done this many times. Stay at one tower and use the lounge in the other tower, no problem at all.

Personally, I like the back tower (Terrace Wing) lounge better. It's much less busier, food and beverage offerings are the same and staff rotates between both lounges.