Mattress on top of another mattress

I have a latex mattress and topper supplied by you (and it is excellent!). I am due to have a hip replacement and my bed and mattress is too low for recovery after this operation. I have a spare 'pocket sprung' mattress and I could get the correct height if I use both mattresses together. Would the mattresses be damaged by doing this and should I put the latex mattress on the top or bottom? By the way I might need to put a board between the mattresses if they dip too much when I sit on the edge. (The height of the bed when I sit on it is the important thing after a hip replacement operation!