The outlet pipes would not tee back together under the floor. Otherwise none of the fixtures would stop running if you turned off just one heater (either heater).
The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit.
having 2 w/h's in series i believe is the most energy efficient.very easy to tell how. c/w in h/w out to c/w in on 2nd w/h then h/w out.The #1 w/h is the work horse it's the one doing the most heating the 2nd w/h is basically a storage tank but will heat if needed.just having a bigger on will help but 2 in series is the way to go.