Different Statement of Purpose to the same advisor in two programs?
Yes, it's totally fine. Its absolutely normal to apply for multiple positions and obviously promise them that you will do things that are unique to them if you are accepted. This shows that you have twice the interests and also ideas on how to fit in aside from determination. 

1) Do I need to talk about my research interests in social psych in the SOP to Quantitative psychology?  Yes, as much as you want to mention possible cross-disciplinary researches but not too much. This should be 10-20% of your SOP. The rest should be really aligned with requirements of each program and the possible dissertation committee members from each group. Think of your dissertation topic and board members in each scenario and write for them as audience and signer of your dissertation. 

2) if I do, what's the best approach to avoid my adviser feeling confused about my research interests? SOP is not a marriage proposal. It shows that you have interests in both fields and work and degrees. And you really want to be in that school. He will not feel betrayed or anything. If anything he should see how much you are interested in the whole field. Don't deny that you have skills in both fields though. There should be some 10-20% overlap. but you are proposing a whole new direction and possibility of your dissertation work. 

Big assumption: universities don't really want you to be firm on your research interest when you are not even accepted yet. And even after that there might be no funding to some topics, so you might have to do other stuff. Research is just another industry that is limited to its resources and can not be idealistic about constraints. Meanwhile, they love to see applicants with muti-dimensional skills sets and openness to cross-disciplinary research. But beware of not looking focused on each of these SOPs.