Thin black plastic strip on bathroom trap?

Plumbing Pros (or DIYers), I recently completed a renovation to a new bathroom shower that has two shower heads - one "normal" wall one and another hand shower on a holder. I ran 3/4" PEX up to the bathroom zone, then teed off to the hot and cold shower feeds with 1/2" PEX. I opened up the 1/2 PEX runs and confirmed that EACH side (cold and hot) were delivering well over 6 gallons of water per minute.... PLENTY I figured for two heads that would each probably draw max 2 GPM each.. right? We'll I hooked up a Delta R10000-UNBX rough valve to control water, then above that a 3 position diverted... hand, top, or both. Everything works as expected, but when I have BOTH running pressure is noticeably less.. One or the other is fine.. just not both running at the same time.. SO... what did I do wrong? I NOW know that there is a higher pressure valve that Delta makes, but is that the answer from the start? I have a normal "14" trim kit installed... would upgrading to a "17" add more flow? Is there a new cartridge only or something I can DO to the existing cartridge to improve flow? Any ideas? I don't have access to the valve body anymore.. behind tile from one side and studs from the other.. no no way to change out the rough valve.. just want to increase pressure. Thanks in advance! Jay

Is there any reason I can not install a double san tee on a kitchen sink drain line? The idea is to use the second half of the San Tee as a clean out along the horizontal line. I have a 2 inch vertical that runs to the roof and vents, with 2 kitchen sinks that drain in to it with a san tee. thanks,