Hi Michelle,
You're probably better served by speaking to a plastic surgeon about that. I had under-muscle implants, and the separation between the breasts is dictated by where you chest muscles attach to your sternum. There's not much you can do about that, I would think. There might be some leeway for sub-glandular implants, but again, I'm not one who would have definitive answers.
I went with larger implants, and I can get some decent cleavage with a good bra.
~Terri