If you are going to use preprovisioned mode, you can assign only one routing-engine to the VC. But this is something that you should avoid. If the only routing-engine will fail, you will end up with a FPC that is in linecard mode. This is something that you don't want.
If you are going to use a non preprovisioned mode, then there will be an automatical election and, in your case, both FPCs will be in routing engine roles. One will be the master and the other one the backup.
As you can see, a routing-engine is just a smarter linecard, so preferably is to have as many routing-engines as possible. Of course you are limited to only two routing-engines:
[edit virtual-chassis] 'member' No more than 2 routing engines can be configured error: configuration check-out failed
If this worked for you please flag my post as an "Accepted Solution" so others can benefit. A kudo would be cool if you think I earned it.