Sounds like you need the Japanese futon life. No frame, so there’s no weight limit. Just two futons, when not in use you fold them and stack them. More on reddit.com

Does anyone have experience of trundle beds - intention is to have one like this in guest room, for use by adults visiting or the occasional time when... More on mumsnet.com

The typical daybed or trundle can take a mattress that is 8” high on the pop up trundle and still store properly underneath. You can push the thickness slightly but you will have to remove the bedsheets when you store it. This is a daybed that my family’s bed business makes. It comes in two sizes and accommodates a pop up trundle. The Chelsea mattress is made especially for daybeds and trundles. https://www.charlesprogers.com/Campaign-Daybed https://www.charlesprogers.com/Chelsea More on reddit.com

Late post, but im doing it for the archives and google searchers. I have looked at every corner of the internet and spent 2 weeks nonstop looking at daybeds for my guest room. Birch Lane (higher end brand of Wayfair) is the ONLY strong solid wood day bed I found w trundle (unless you spend $$$$ w an Amish store). The SKU is CDCH1056. It's weight capacity is something like 700 lbs for the bed and 300 lbs for the trundle. Pricey at around $8-900 but again, it's the only bed you can find online boasting that weight capacity. Everything else is flimsy fake wood. I also liked that the bed can take 10" thick mattresses while the other flimsy daybeds only took 8". Good luck More on reddit.com