Are you talking about two bridges side-by-side (i.e. parallel), or end-to-end?
If you are running end-to-end, then two end abutments and one central pier.
If you are running side by side, you can place individual abutments under the ends of each bridge or fabricate one large abutment. Well, actually you'll need two, or do you.
What i've found in all of my bridge travels is that as long as you follow a few proto building techniques like using stone, concrete or wood for your abutments, what you come up with to support your bridge's ends will end up being prototypical.
also, don't be afraid to mix abutment materials. I've seen one abutment being made of cut stone and the other of poured concrete.
The following pics are of a deck plate girder bridge, but the original was a truss. If you look real close through the trees you can see the remains of an original stone abutment (nice detail touch).
Actually, further south down the river in Springfield there is an example of twin truss spans running across the river. However, I think that each span is single track.
I have to go by there today, I'll see if I can catch some pics. I was planning on doing that in the fall after the leaves fell, but... in the name of MRRing, I'll do it.