To answer your first question, it is not possible to configure two bridges on the same firewall.
The question I have is, what do you mean asymetric traffic? Are you talking about asymetric routing, where the return traffic would get routed through another device? This can be allowed through the firewall by turning off stateful inspection, but you do lose the ability to do any inspection.
I recommend that you take a look at the 8.3.0 Product Guide section on "Transparent (bridged) mode deployment".