them "front to front."  There are more possibilities for me, both as a participant and a voyeur.  If you book them separately, there is only one combination at each session.  If you book them together, there are four different kinds of physical engagement possible, which makes for a more adventurous session.   Just my opinion.  Others may disagree.  

But depends on what you mean by back to back...  

 
They were my two consecutive appointments.  They weren't on the same  day but separated by couple of weeks.  I met the roomie when I went to meet the girl I was initially scheduled to meet, and really liked her  So I went back and saw her a couple of weeks later.

If I book people, then I book them at least 60--70 minutes apart so that I can get things together and so they can get to where I am (Because I do In-call).