and it wont make it structurally less sound?
I would really suggest getting a design book from your local hardware store to figure out the spacing. The one that I have shows the longest spans that you want with beams is 12'. So if you have a 16' span, you'll want to set a post in the middle. With the post in the middle, either two 8' or one 16' really doesn't matter as long as that split is at the post. In order to make that connection, you may want to use a 4x6 post to help give room to make the connection for both of the 8' beams.
If by supports, you mean the joists then you can do a 16' span with a 2x10 if you space the joists at 16". Once again, there are several considerations with this including the species of wood and the decking you are using.