Hi JoelD,
Hopefully some of the members here that are using two twin XL’s as a king size mattress will see your post and comment but I think it’s probably fair to say that most people would feel the split in between the mattresses … particularly if they use or sleep in the center of the mattress or with some of the other activities that take place on a mattress. It will also depend on how flush the two mattresses fit together and different people would of course have different opinions about whether the tradeoffs were worth it so I would also be very interested in any feedback from members and how they feel about the tradeoffs involved.
Post #8 here also has some information about split mattresses and options that can mask the split if you do go in this direction and decide that you don’t like the feel of the split in between the mattresses.
Phoenix