Is the symptom you are seeing show up as the inability of the system to produce enough hot water? Many gas appliances operate internally at less than a pound of pressure...the in-line regulator adjusts that so 2 on the inlet might be fine.
If what you are seeing is not enough water pressure, or the water not hot enough, there may also be a volume restriction in the tankless unit. It can only supply so much volume while producing a suitable heat gain. On a multi-head shower, the unit you have may not produce enough volume for the shower you want - it should work in many places for a single, restricted (per today's codes) head, though. You may need yet a bigger gas line and a second, in-line tankless to produce the volume you want.
How many GPM are you trying to achieve, and how cold is the incoming water?