There was much experimentation with compound set ups. Most used either two high pressure and two low pressure or two high pressure and one low pressure. The latter was use by Webb on the LNWR. He had the two sizes driving different axles with different valve gears and no coupling rods. In certain circumstances this could lead to the two sets of driving wheels turning in opposite directions!
There were also several other experimental types, such as one that had two different cylinders in tandem driving a single piston rod, ones with one high pressure cylinder on one side of the loco and a single low pressure one on the other. Then there were other schemes that drove two axles with the cylinders placed between them, indirect drives that drove another shaft that was in turn connected to the wheels etc.

Robin