setting up external NLB with more than one ISP

We have 2 TMG 2010 servers in an Enterprise Array, each have 3 external NICs, 1 DMZ NIC and 1 internal NIC.
    - NLB is setup on the internal NICs
    - ISP Redundancy on External NICs B & C
    - NLB is setup external NIC B - 12.??.??.??/25 IPs for published web and ftp sites. 

We would like to add external NIC A (209.??.??.??/27 to the external NLB.

How can I do this?  Could I setup a netmask on the primary VIP to be a /27(225.255.255.224) if we use less than 30 VIPs in both the 12... and the 209... subnets?

It seems like this should be do-able, maybe not as I describe.

Thanks,  Mike