I dont know of any pto pumps that are two stage altho it should be possible to build one. Pumps displace fluid according to the displacement area of the pump. The area is usually rated in cubic inches. A 1 cuin displacment pump will move 1 cuin volume of oil per revolution of the pump shaft. There are 231 cuin in a gal, so turn that 1cuin pump 231 rpms and it will pump 1 gpm of oil. With that known, you could possibly take a 28gpm two stage wood splitter pump, that is designed to run at 3600 rpms and run it behind a 1000rpm pto shaft and end up with two stage pump that produces around 7.5gpm of flow. This 7.5 gpm of flow would be the combined flow of the high side and the low side of the pump. Once pressure builds enough to dump the high flow and produce high pressure, the actual flow to cylinder would be much lower. In other words, your splitter would be pretty slow.
Lets say you do find a 2 stage pto pump for your wood splitter. You said your splitter wont always split some of the big wood. The problem might not be the pump. Your relief could be set to low not allowing the pump to build the full pressure it is capable of producing. If this is the case, all you might need to do is adjust the relief a few turns and have a power house of a wood splitter. You really need to install a gauge to see what pressure you are building when the splitter cylinder stalls and then see if the relief can be adjusted to allow more pressure build up.
Another possible factor of low power is cylinder size, if your pump can build the pressure and the cylinder still wont split the wood, look to see what size cylinder you have. Splitting power is a direct relation of pump pressure and cyl piston area. The larger the bore of the cylinder, the more force it can apply. Check your pressure and measure cyl bore before buying pumps or parts.
