Kelly
I saw a peri in conjunction with my regular OB. I saw the peri every 6 weeks and she did US at each visit to check Logan's growth. I continued to see my regular OB every 4 weeks. When I was sick, she worked with a local pulmo doc (kinda complicated story but the short version is that I was still seen at the local CF clinic which was a pediatric clinic and they were scared to treat me) to prescribe appropriate medications safe for pregnancy. I didn't use the peri for delivery though - she was way more into medical birth than I wanted plus she only delivered at the large local hospital and I wanted to deliver at the smaller more baby friendly hospital. She never expected to deliver me though. I do, however, think she was brilliant and really liked her. I'll be seeing her again for my next pregnancy.
If you like your current OB then by all means just do tandem care with a peri and your regular OB. Also, I know its early in the game, but don't let them pressure you into a scheduled c-section if that is not something you want. Lots and lots of twin moms successfully deliver vaginally. Start educating yourself about twin birth now so you are prepared later to discuss your birth plan and options with your OB(s).