[Will]

No need to reinstall the heads. Zero compression = a huge leak, it should be pretty obvious what's going on by physical inspection.

Set the head up so the deck surface is on top and level, then fill the chambers. If there is a big enough valve leak to cause zero compression, the water will pour out of whichever port has the problematic valve.

Alternative idea - pull the valves out of the heads in those two cylinders and visually inspect the seats and valves. If nothing is obvious, get some machinist bluing, spray the seat area then whack the valve against the seat by hand. The impression left in the bluing will show if the valve is not making good contact with the seat - the impression should be uniform all the way around and leave a uniform blue band on the valve also - and the mark on the valve should be on the seating surface and leave a small margin around the outside edge of the valve.

Doing this will also reveal if you have tight guides or bent valves.

Check your pushrods. Are they all straight? Any damage to the ends where they sit in the lifter or the cup in the rocker arm? Check the cam, how do the lobes and lifters look?

If all that checks out, then the problem has to be with the pistons & rings. Bring a piston up to TDC and see if it walks/wobbles in the bore. How do the finishes on the cylinder walls look?

I've worked on engines with blown head gaskets, burned valves, and cracked rings/ring lands and they always showed at least some compression on the problem cylinder and the problems were visually obvious. Zero compression means a BIG problem that has got to be visible.