I think that even if you have a posterior presentation with a second baby it is more likely to turn during labour and so not actually be posterior at time of delivery iyswim
ime DD1 was posterior and a full back labour and I tore (she was 8lbs) DD2 was posterior but turned during labour and I didn't tear (she was quite a lot bigger too at 9-15)
Not sure about the presentation but I really believe that delivering in water with a mw who makes you stop and wait and breathe at the crucial moment can help with the tearing (lack of I mean )
good luck