In our guest room, we have a twin trundle bed. When I pop up the twin trundle part & smoosh it against the bed, I create a king size; the trundle's headboard becomes the king headboard. I added a foam eggshell to the trundle part to equalize the slight height difference, and bought a thick, comfy king-sized mattress pad to go atop the eggshell & both mattresses. This pad holds the 2 twins together; the fitted sheet & usual bedding go atop that, & we've never had a separation problem (the trundle's on rollers, but it's on a carpet & the rollers only roll towards & from the main bed, not side to side). I've asked relatives to be very honest about this setup, and both my BIL/SIL and my brother/SIL have said it's an incredibly comfortable guest bed, no complaints (both mattresses are good-quality, too). I've tried it out also, & you really can't feel the seam, since the height is equal & it's well-padded.
Does that help?
If you're afraid of butter, use cream. ~~ Julia Child
As you cook, you enjoy omniscience about food that no amount of label reading can match. Having retaken control of the meal from the food scientists, you know exactly what is in it. (Unless you start w/cream of mushroom soup, in which case all bets are off.) To reclaim control over one's food, to take it back from industry & science, is no small thing; indeed, in our time, cooking from scratch qualifies as subversive. ~~ Michael Pollan