H and I beams are rigid in bending but about worthless on their own in twist
if you can, you might consider welding in round section crossmembers between the two beams
this will dramatically stiffen the beams from twisting.
you wanna make sure everything is plumb and level before you weld them in, because afterwards the beams will not
twist any appreciable amount (which is what you are after anyway)
torque tube, torque box construction is very rigid and will keep thing from twisting under load
it is this twisting too and froe that makes the drive coupler play hell with its spider element
if you go the route of round crossmembers, remember to make them as large in diameter as you can
after installation the tubes will maintain the beams from twisting, because to do so would require the round crossmembers
would also have to twist, which they will not to any appreciable degree if they are reasonably large in diameter.
bob g