Sadly, Phishfood, it is one of the plumbers from whom I am soliciting bids that has told me I cannot hook them up in parallel. It was another one who suggested it in the first place, hence my confusion.
There certainly seems to be enough room to accommodate an additional heater - utility room has two outside walls for venting. A 1-inch black iron pipe supplies natural gas at city pressure [ 4 oz I believe ] but local utility indicates they can bump that up at regulator, if necessary. House load is currently 290k btu, between cooktop, two furnaces and fireplace, but local utility also indicated that wouldn't be an issue.
Wonder if parallel installation of new heater next to existing one would present problem balancing the work load. Seems if there was sediment load, pipe scale or blockage, one heater would be doing all the work and situation would not necessarily be improved.
Maybe a heater set to 100 in series in front of the existing heater would be best path, cutting down on recovery time during high demand. City water comes out of tap at 51, so it's a heavy lift to get to 120.
Grateful for any additional thoughts.