Help required in configuring two IPs for the one Interface

Hi All you Intelligent ppl.

I desperately need some suggestion rather solution to one of my problem for installing a lab topology in my office.the scenario goes like this.

We have four router here in our LAB.Two 7200 and two 3400. What I want to achieve is the two 7200 router will be in the middle and will act as SSG ( you all may not be aware of this term,but it doesnt matter) and one 3400 will act as a client which will be @ one side of the topology and the other 3400 will be a service router which will reside at the other end.Now my problem is both the SSG router is connected to the client router through only one ethernet interface ofcourse one switch in-between.So only a single IP is configured at the client router for both the SSG router.So its creating a problem in using both the SSG router at the same time.So I tried with configuring two different IPs (with the help of subnet masking) for the same interface in the client router one as primary and one as secondary.But this didnt help in resolving the problem. Ofcourse we have limitation here is we cant configure two ethernet interface for each SSG router with two different IPs.

So please suggest me some solution to this problem or some other approach which I can follow to set up my desired topology.If any routing protocol we need to configure for this case.
Will Highly appreciate your time and effort.

Thanks
optimos