Breast Augmentation with Prominent Sternum?

Your condition is called Pectus Carinatum ( latin for "bird like" chest")
A lot of good advice has been given on this forum, I would only add that the best option for you will be to use a High Profile implant.
The high profile implant projects more from the chest wall, counteracting the effect of the prominent sternum.
I hope the link I provide below will help you understand the whole issue of projection and chest types a little better. It goes into high placed implants versus medially placed implants, both of which are referred to as having "good cleavage"
You may not be able to get a super defined medial cleavage, but with a high profile implant and proper placement (higher?) you may get closer to what you want.

There's nothing that can be done about your prominent breast bone, at least nothing reasonable. You need to have a standard breast augmentation done and your breast bone will still be prominent. If your nipples point a little to the sides, they'll appear to point a little more towards the sides just as a natural effect of the surgery. Remember that the augmentation will enhance every aspect of your breasts (both good and not so good). Ultimately, you'll still be very excited about your result since the augmentation factor is no longer of concern.
You can't really place the implants any closer together if they're being placed under the muscle. You don't want him to split the muscle along the center just to satisfy you insisting on cleavage. I also don't think you should get a bigger implant to cause cleavage. Cleavage is usually something you see when someone has a bra on and it's pressing the breasts together. It's not really because of a certain style, position or size of an implant -- so you should reconsider changing your implant with that intention in mind.
You should revisit with your ABPS board certified plastic surgeon on these issues and clarify your expectations so everybody is thrilled after surgery.