Combining two water inlets into one. Does increase pressure
Here are some basic fluid dynamic examples that should help answer your question:

With no flow, the pressure is the same everywhere. If the capped pressure is not the same at your two destinations, that means that there is flow in the city main (left on my diagrams) and there are significant losses in the city main (may be undersized, nothing you can do). 



As the water exits an open pipe, the pressure is zero. The flow rate depends on the losses in the pipe and the pressure difference. In the college Fluid Dynamics class I took nearly 40 years ago, we used published graphs for pipes to determine flow for pressure and length. There are probably computer programs and/or online tools to do the same now.

 

Combining the two pipes will double the flow rate, assuming that there are minimal losses after the pipes are combined.



If two pipes with different static pressures are connected, water will flow back to the city main. Assuming that your question is real, and that the city will allow you to do this, at a minimum they will insist on back-flow preventers.



If the pipes are not capped or open, if they are connected to a small nozzle, the solution is not trivial anymore. The best thing to do would be to add pressure gauges to see what the destination pressure is.