Yes you can
you can even enable ip_forward on the proxmox-host and route between differen vmbridges and their specific IP-subnets.
But be warned - we had one issue setting the whole VM offline:
We connected vmbr0 to the vm (KVM based) and all of a sudden - this box that was available for months was offline - debugging showed the MAC# of the virutal ethernet to show up on the vmtabXXXi0 interface not on vmbr0.
One reboot of this VM and it's mac was visible on vmbr0 again...
I have no specific idea how and why this happened though.