Can a tankless water heater outlet pressure be increased without a second heater?
sounds like you want more volume (gpm) of hot water ( thus,two tankless in parallel) because it takes energy (Btu) to make hot water. The heater will also limit its gpm output into the water pipe by the temperature you set.
Look up the volume to hot water temperature curves for your rated Btu tankless heater.  I think a big 200,000 Btu units can provide 6gpm of 115 F hot water and a smaller 60,000 Btu heater may only do 3gpm of 115 F hot water.
If the kitchen is asking for all 6gpm of hot 115 F water, then all the other valves located after the kitchen valve will have no hot water from the water line.
The 200 MBtu will make 7 gpm of warm water if the temperature is turned down (check the 90 F curve). Now, the kitchen gets 6gpm of 90 F water, and 1 gpm of 90 F water can go down the pipe for the next fixture.
System with multiple loops of heaters installed closer to the fixtures, will fire on independently with no pressure lost. Parallel heaters will use more Btu and water if the the last fixture and heaters are far apart.
These heater also have a minimal in coming water pressure to operate correctly, so a pump may help if placed before the heater.