If I was invited to a double baby shower I would feel badly if I was given an invitation with only my friends name on it. Put both girls names on ALL the invites. People will figure out on their own if they want to bring a gift for just the girl they know or both. I personally would bring a gift for both. All babies are worth celebrating and I don't think any one will feel put out if they "have" to buy another gift.
Games will be hard with 50 people, and would probably take forever. For my first baby shower my mom and sister bought a white piggy bank and some markers meant for glass. They put my daughter's name on it and put it by the door. Everyone that came either put their name or drew a little picture. I loved it and it sits in my daughters room still. If these are first time mommas an "advice/story" book would be fun and easy. People could just write in it as they wished. You could also set up a station with white onsies and fabric markers and people could decorate a onsie on thier own throughout the party.