as far as i can tell, they don't care what you install something on. possibly if it is the cause of the problem you call for (i.e. incompatible hardware) they would care. at least that's the way our licensing seems to work ... i don't call and say "server xyz is having a problem," i call and say "i'm having a problem with satellite" or whatever product.
if you plan to run the desktop after you have the two servers installed, you will have to disable updates on the desktop to free up the two entitlements for the servers. if you plan to turn off the desktop at that point, obviously this will not be of issue.
finally, rhel workstation is pretty much the same thing as rhel servers. both run just fine on desktops. there are no magical chips that flag the install saying "this is a server" or "this is a desktop." like i said before, i use advanced server on my desktop at work. it works just fine.