Uplink Balancing - two routers on same subnet

Hello,

I am trying to configure our UTM to work with our dual WAN office routers.  We use BGP multihoming for 2x Ethernet WAN connections from 2 different ISPs.  Right now, the routers are configured with Cisco HSRP, advertising a single virtual IP address of which our UTM has set as its Default Gateway.

But, I'm more interested to see if the UTM can handle the uplink balancing.  We do have a switch between the UTM and the routers, and so each router has a physical interface to the switch and there is a single physical interface from the UTM plugged into the switch.  Both router interfaces are in the same VLAN and same subnet (as is the HSRP VIP).

What I'm struggling with is, I'd like to setup Uplink Balancing but it seems I cannot do that with my two router interfaces.  The router interfaces are 192.168.255.2 and 192.168.255.3, with the HSRP VIP being 192.168.255.1.  The UTM interface is 192.168.255.254 and its GW is set to 192.168.255.1.  What I'd like to do is setup uplink balancing on 192.168.255.2 and 192.168.255.3, and then I can remove the HSRP VIP.

Soooo... how do I configure uplink balancing in the UTM when both uplink IPs are on the same subnet, and I have only one physical interface from the UTM going toward the WAN?  I think what I want is to balance uplinks based on IP address, not on interface..

Or, Is this called multipathing?

Cheers

SAM