It was an attempt to split the piston forces and resultant hammer blow of a given wheel arrangement and allow for smaller reciprocating parts, albeit while having to maintain two sets of cylinders and a lot more plumbing.

Thus you get a split 4-8-4 as a 4-4-4-4 and a split 4-10-4 as a 4-4-6-4.

The Q1 was a one-off. The unfortunate placement of the rear cylinders at the rear of the second engine put them and the crosshead at the dirtiest and most cramped location on the locomotive. It also restricted the firebox size. That's why the Q2 made the front engine the 2 axle one and put the three-axle second engine the right way forward.

The Q2s were an attempt to improve upon the J1 2-10-4s but they were only marginally better, even when they weren't slip-sliding away.

It would have been interesting to see what the Pennsy would have come up with if they had built R2 class 4-8-4s instead of the exotic T1s.