For 20 years we've used two twin beds inside of a king bed frame. It definitely made them easier to transport, and do maintenance flips*, and with a memory foam topper the crack in the middle wasn't perceptible, but it turns out that two twin mattresses are as wide as a king mattress, but are 5 or so inches shorter than a king mattress, which is why we've always had trouble with the sheets being very wrinkly.
Our new bed is great, and the frame we put it on is very strong and simplifies things quite a bit (no more box springs, lots of underbed storage space). I'm also happy because I no longer have to deal with the mattress business and their horrible prices and the fact that you can't comparison shop because every manufacturer makes a different model for every store.
* for those that have to flip mattresses: I marked the ends of the old mattresses with the numbers 1-4 so I'd know which way to flip the mattress, 1 and 2 on the top and bottom of one end, and 3 and 4 of the top and bottom of the other end. So if you were at 1, you flipped the mattress so that 2 was in the same spot, etc. That assured that the mattress was flipped upside and end for end as necessary. Some mattresses today don't need to be flipped at all, or only need to be rotated end for end and never flipped over; see your label or instructions for more details.