We have decided to upgrade to a king size bed, however, I really do not like the motion transfer we get with the innerspring mattress.  I cannot go with latex or other foam based mattress due to back pain, so we are stuck with innerspring.  Pocket coils are out too ( have one and it does not hold up over time).  Some posters on this forum seem to think a king size innerspring mattress will sag in the middle due to its construction.  I've never owned a king size so I'd like to hear more about if it will sag or not. 

Anyone who has used two twinXLs as a king, I would really appreciate it if you would share your experience as to how did it work out, was it comfortable for both, did you fall through the crack, and would a bridge (FBM has a foam bridge) make it feel like one mattress?  The mattress is going on top of a king sized platform bed, no box springs.  Thanks in advance.