I'm currently in the middle of a ground up resto/modification of an 86' F250.  I'm not building it to be a daily driver, but still need it to be street legal in order to get it around from trail to pit to trail.  I've recently aquired another chassis exactly like the one I'm building from and am contemplating marrying to two together for a couple of reasons.  First, is for strength while still allowing for some flex.  Second is the great ground clearance/protection to the driveline since it would be mounted to the top frame.  Third, I'll be able to get 9 inches of ground clearance with only a four inch suspension lift.  And last, just because I haven't seen a lot of guys do it.  Has anyone on this site done it before or have any input on the matter?  

are you talking about stacking frames??? setting one on to of the other... and mounting the body on the top frame, and the drivetrain on the bottom frame??? there's a website called "scary lifts"... check it out... i'm sure someone has the website address... it's not only logistically a nightmare... but it's structurally unsound... you may as well do an 8 inch body lift... you've got two frames which flex on seperate axis and will therefore flex against each other... so any way of joining the frames will be compromised and will fail... you'll end up with a body and frame laying on the road and a drivetrain and frame driving away... and that's a best case scenario... it's not plausable... or even a good idea... if you're going to do it... do it right... there are better... more reasonable way to go about lifting the truck and gaining ground clearance without "rednecking" it to such an extent... do some research on portal axles... just do it right...  

I agree, stacking frame is not wise and if you want to beef it up, box the existing frame.  

I suppose I's looking at it as "thinking outside the box".  Keep in mind I wasn't planning on just glueing one frame on top of the other.  I was actually putting a lot of thought into ways to redesign the cross members and what modifications would have to be made to either frame to make it all work allowing movent of the new structure to keep everything from cracking.  I've witnessed projects where people have done it in the past with good success, but admit it is a scary challenge.