LOL here! I can't believe I stumped YOU

! Now I don't feel so dumb.
After some more troubleshooting this morning I resolved the shower and it is working.
First of all I still don't know why turning the sink faucet on added more pressure to the shower. I did figure out the because the cartridge was a moen with the posi-temp that I had to set the temp with the new cartridge. Previously I skipped this step in the directions. When I went back over directions I was able to get the correct water flow and temp. Shower fixed.\
Thanks for helping me. Plumbing is a lot of deductive reasoning and patience.