After having another look at my control valve I would guess the centering springs are causing the stiffness.I don't feel any friction or binding in the linkage,just the lever returning to neutral with quite a bit of force.If it is caused by stiff springs,is there any thing that can be done about it or will it loosen up over time with usage?To the OP,I am not trying to hijack your thread,just trying to find a solution for everyone.Are your valves cable operated, or lever operated?
If both spools have about the same resistance, the I would think the spools are tight due to the centering springs used.
You might remove the spools and see if there is any debris in the spool.
Are the levers any different from one valve to another, or same length, etc?