If we use THK/NSK/ect. style rail for the x-axis on a light etching machine, can we use just 1 bearing block per X-rail to support the gantry or is there too much play, so that we'd need 2 bearing separated apart a few inches to keep the gantry vertical? There will be very negligible contact force, just the weight (50 lb. max) of the gantry.
You will have a severe racking problem if you only have one bearing per side. The cutting forces will make whichever side of the gantry you are cutting lag the other side.