Is it dangerous to take Zoloft 45 and Paxil (paroxetine) 20 if I half them and take it together to transition? Dr didn't tell me anything about serotonin syndrome.

Long half-life. Both of these medications have a very long half-life so they will be in your body even if you stop them abruptly. Also, meds take ~4.5 half-lives (with regular dosing) before they are "steady state" effective (it depends on the exact drug & its "kinetics"). Less than 0.1% get serotonin syndrome (hard to say who). No prob stopping 1 SSRI & starting the other. Follow-up with your doctor/psychiatrist.