Open center hydraulic: Pump creates flow, which always goes through valve and back to tank. When a valve opens this flow is redirected into that function, if there is resistance to that flow then a load is on the system (such as a cylinder lifting or reaching end of travel) pressure builds and the "relief" which is a ball on a stiff spring releases and diverts to the tank. So the relief keeps the system from building more pressure than the pump (or hoses, or cylinders, or valves, but especially the pump) can withstand. So to "deadhead" the system is to do something to make the pressure build until the relief valve open, usually run a cylinder against its end stop. Can be deliberate (as a test) or not.
Closed center: Pump creates pressure, and when pressure drops changes to create volume. This system closes all the valves off to retain pressure when flow is not needed.
Some of the fun comes when a hobby builder scrounges a closed center valve and uses it in an open center system, then it is building pressure all the time when it does not need to.
When running two sets of valves on an open center system, there are a few ways to do this.
One of the best ways is to run a separate pump for each valve set. This can be mounted separately or all part of one unit called a multi section pump, but each section is still basically a separate pump.
Another way is to use something called a rotary flow divider. This is basically a multi section pump that is driven by the hydraulic pressure, and geared together. Usually 1:1 for each section (I have not seen more than two but that doesn't mean they aren't out there). What this does it makes sure equal flow goes to both outputs without losing pressure.
Yet another way is a pressure priority valve. This acts like a flow divider crossed with a relief valve. It uses a series of spring valves to resist the pressure and basically lets one function steal all the pressure before the others get to use it.
If you just tee the output output to two open section valves, whichever one that is not being used steals all the flow and the one you are using never builds pressure and causes the motors or cylinders to move. So you could "cheat" the system and put an extra pressure relief in line with both valves, and plug the far end to basically turn it into a poor man's closed center system. These relief valves would be running all the time, putting extra load on the motor and heating up your fluid.
This sort of sounds like what your system is doing, based on what you have described.
A "power beyond" valve set up gets around all of this by running the valves in series. It is a special setup to let the next valve in line build pressure through the first one while it isn't working itself. This also means that any time the first one tries to do something it steals all the flow from the 2nd. But it is a very simple and economical way to build a system with multiple valve sets.
I want to say that the "standard" cadtrac plans call for the use of a rotary divider. The price of adding a rotary divider vs. using a multi section pump is very close, and the multi section pumps seem to be easier to find.
It sort of sounds to me like whoever built your system tried to cut some corners on the build, then didn't like the results and sold the unit.
Did you get the plans along with the unit? I don't know if he sells just the hydraulic plans separately or will offer to help diagnose a machine someone else has built and then kept the plans for, but how much have you spent on pumps so far? It might be worth buying the plans and going through the system.
https://cadplans.com/products/cadtrac-1500
