I'm not sure how to overclock them separately. But my guess is that they are incompatible. I'd suggest trying to sell the 1.35V for half the original price you paid and get a second 1.5v as you already have.
Well I think I've seen an option in my bios on an matching pair of old ram that has an option to change the timings of each one differently? Though I may be wrong on that notion, and cant clock these separately? My bad human memory, i cant remember if it actually let me clock them separately. hmmmm.. and then if so would it even run dual channel?
Though If this is not possible I think I will end up returning the 1.5v stick since I still have a week to return it. Since I think the 1.35v one is a little better. I still want to know if this is possible because I would have to pay return shipping
and would loose a few dollars