I have a overhead valve spring compressor with the 2 fingers but it doesn't grab the inner spring. The locks won't release with the inner spring tension on the retainer. I also made a shaft mount type compressor and I bent a stem putting too much force on the handle and it moved sideways slightly. I had a shop go through the heads and replace the bent valve and put in new seals.They measured the pressures of 190lbs on the seat and 450lbs open. I like the rope idea and never seemed confident in the compressed air method of holding the valves up. Has anyone had any luck using the overhead compressor with dual springs? If so, what's the trick?

I never liked to use compressed air to keep valves closed.
All it takes is one sticky valve retainer or lock and the valve only has to open just barely and all the pressure blasts away at once. Just at the moment you were pushing on the retainer to free the valve locks.

As for the cylinder at TDC... Also think of what will happen when the piston is just not quite exactly on TDC. The air pressure will push away the piston and rotate the engine with a sudden jolt.
You should really need to lock the crankshaft in place when doing this and 90-degree marks on the balancer are very helpful.

I've personally changed springs by feeding a length of thick rope into the cylinder through the sparkplug hole.
Not perfect, and you shouldn't lean on the valve retainer too hard with chance of bending the valve head, but at least imo you have some more control over the situation.