bunkclimber posted this a couple years ago. Maybe it could help.
I wanted to do the same thing, add a top clamp to a fork carriage on my 977,which is a two valve machine. The loader hyd pump output is like 80GPM or so with 1-1/4" hose with a Code 61/62 flange output. .you could intercept the flow from the main pump, use a flow divider to feed the Cat tank and loader valving on one side and also a satellite valve to run your outboard implements, with a appropriately sized relief valve and secondary return line back to the Cat hyd tank. You're talking BIG valves, not easy to find, or for cheap. This is what appears to be done with the 977 in the photos of the Montreal Tractor unit you saw. Whether or not they used a relief valve back to the Cat hyd tank in addition to the secondary return or not is to be seen. I've seen a couple machines set up like this for oilfield work many years ago with multi-spool valves next to the operators station and custom outboard tools for pipeline work. .Pressure doubling could develop between systems and make for a catastrophe when used under load without the appropriate relief valve. For the easiest route just get a Cat tank with the factory valves in it and plumb it from there. Best of luck.