First thing to do is actually determine what your water pressure is in the house. You can buy a screw-on pressure gauge or less than $20 and check that.
If the two lines come off of a larger line separately, then you can probably increase the volume available, but that may not change anything in a shower.
Do not confuse volume with pressure. A fire hose and soda straw might have the same pressure, but the volume would be way more on the fire hose. As long as you have more volume than required, more won't do anything for you.
A showerhead by federal regulations, is not supposed to use more than 2.5 gpm. You should get a big bucket and actually measure what volume you get out of the head. IF it's lots more than 2.5gpm, first thing I'd try is to replace the head with one that meets today's regulations. Just like if you try to run multiple sprinklers off of the same hose, the pressure will be the same, but the volume available will be the limiting factor to making them perform. A 1/2" valve is usually capable of about 5-6gpm, way more than a showerhead is supposed to allow, so increasing the volume available won't do a thing.
Dynamic pressure differs from static pressure. The dynamic pressure will often be less than the static pressure because of friction. The more elbows and length, the more dynamic pressure loss. The faster you try to flow the water in the pipe, the more friction, the lower the dynamic pressure.
So, two tasks, see what your static pressure is, then see how much volume comes out of your showerhead, then report back.