You can open the firewall to allow access to the proxy between networks. Another solution is to add multiple network adapters to the Balance VA, and put them on each VM network, if there are not too many of them.
Indeed and opposite to products collecting VM only through vCenter, OC Balance collects data directly from the Windows VM, allowing Balance to get VM paging, swapping, check alignement and draw end-to-end topology. Moreover, thanks to this end-to-end monitoring of the data path, Balance can apply its analytics to detect bottlenecks or rogue VM.
Without the Balance discovering and monitoring the VM, you're loosing many unique Balance features I'm afraid.
Windows VM are collected and monitored by the Balance Proxy.
Linux and Unix VM are collected and monitored by the Balance server.
The connection between the Balance Server and the Balance Proxy is through a single TCP port. What about setting up a proxy with dual network connection and appropriate filtering rules to allow only it to flow between the two networks?