TWO EMPLOYEES WERE REMOVING THE HEAD FROM A HEATER AFTER IT HAD BEEN TAKEN OUT OF SERVICE. THEY HAD CLEANED THE TOP BY BLOWING IT OFF WITH AIR & WERE READY TO REMOVE BOLTS. THEY LEFT FOR A SHO RT TIME & UPON RETURNING BEGAN REMOVING BOLTS. HOT LIQUID BEGAN SPRAYING OUT CONTACTING BOTH. INVESTIGATION REVEALED THEY HAD BEGUN WORK ON AN ADJACENT HEATER THAT WAS STILL IN SERVICE.