I have performed this in the past and while it is possible it is not necessarily adviseable without you understanding the risks, particularly of bottoming out or a double bubble.
There are numerous increased risks associated with this, and I am sure I will not even touch on half of them, but consider the following.
The size of the implant has been associated with:
1.Increased risk of loss of nipple sensation
2.Increased risk for long term breast ptosis (sagging)
3.Increased risk for chest wall deformation (curving of the ribs)
4.Increased risk of rippling or palpable /visible creases
5.Increased risk of lower pole tissue attenuation (thinning of the tissues of the breast)
6.Increased risk of secondary revisionary procedures
These are a few of the risks off the top of my head. Please discuss with your surgeon.
Most importantly, remember that although you are seeking breast enlargement, many women present complaining of breast overgrowth desiring breast reduction. These women report limited physical activity, neck/back/shoulder pain, shoulder grooving from bra straps, numbness in the fingers, rashes beneath the breasts, etc.
Many of these women feel significant relief with reductions as small as 300 cc yet you are considering adding twice that to your breasts. Think it over carefully.
I hope this helps.