Hi I have 5 kids and we cosleep, right now it is 5 in a king size (5 yo sleeps at the bottom under his 3 yo brother, baby next to me and Daddy) but we have also done queen and twin next to each other.
Good for you for cosleeping and not letting the crib manufacturers and medically indoctrinated scare you out of staying close to your baby. Cosleeping is safe if done properly. Those who think otherwise should google "James McKenna" and do some reading.
The problem with two mattresses is more than the gap, it's also changing the bedding! With two mattresses together, especially if you are keeping them together by pushing them tightly against the wall, it is very hard to change the bedding. Also it makes it harder for people to get our of bed, especially mama who usually gets stuck in the middle!
Sleeping with the crack at your knees might not be a bad idea, as long as you have enough room for those who are nursing to reach you without crushing baby or husband. Certainly it would be easier than having to procure pads and things to hold the mattresses together. I've never heard of those t-shape things people are talking about to fill in the gap, they sound useful. I rolled up an old mattress pad and used it to fill the gap, we didn't have a teeny baby at the time to worry about. Covering both beds with memory foam would work, but then you'd have problems again with bedding because even a king size flat sheet probably won't be long enough to cover two full size beds. So you need to plan on using full size fitted sheets. Which may rule out the t thing? You'll have to research that.
To keep the mattresses together without making it too hard to change bedding, I suggest using a strap. Something like seatbelt material, long enough to go around the outside of both beds, with a heavy clamp on it. You kind of pump the clamp to tighten it. Truckers use them and I use them sometimes in my part-time job as stagehand. I don't know where you would purchase such a thing! But it would bind the mattresses together with the sheets on, and would be easy to loosen to change the sheets, and tighten the beds back up by yourself, provided the bed is away from the wall on at least two sides. Or possibly you could get a long belt of that type of material, and attach rings to it like a ring sling uses, and get the same effect, I don't know if you could get it tight enough not to slip using the rings.
Good luck finding a solution!
Oh and if you want check out alternamas on yahoogroups. It's a local group of Memphis mamas who breastfeed co-sleep, etc.