ICS -> Untangle + Bridge to another LAN

Hi, first post guys, so bear with me...

Current Scenario:

Main Office:
WAN: ADSL 512 Kbps/128 Kbps
Untangle: Router mode
LAN: Couple of servers and fifteen desktops (192.168.0.x)

Showroom:
WAN: Fibre Optic 10 Mbps/2 Mbps
Untangle: Router mode
LAN: five desktops (192.168.1.x)

Requirements:
RDP over VPN into Main Office LAN to access dynamic ERP
app(ASP.NET on IIS to SQL Server)

Site-to-site VPN between two UTs (Main Office - Showroom)
works fine, but slow.

Boss hands me a 3G USB modem (21.6 Mbps/~5 Mbps), asks
me to check whether it's possible to establish site-to-site VPN
using that. Reason we can't upgrade our Main Office WAN is
because there's no fibre optic here, and the ADSL upgrade
charges are prohibitively expensive.

So here's what I'm thinking:

Main Office:
WAN: ADSL 512 Kbps/128 Kbps
Untangle: Router mode
LAN: Couple of servers and fifteen desktops (192.168.0.x)

And...

USB modem connected to Win7 box with ICS, connected
with crossover to external interface of another Untangle
box operating in Bridge mode.

Then, establish Site-to-Site VPN between Main Office
second UT box and showroom UT box.

So far, this makes sense to me. The point where I'm stuck
is...how do I make the ICS box talk to the other network in
the Main Office?

Reason they need to talk is because the app/db server are
on the other LAN (192.168.0.x), so it's kind of pointless to
RDP over VPN and have the machine you RDP'ed into not be
able to access the ERP app. ;)

Oh and before you ask, machine being RDP'ed into is patched
for concurrency, so that's not a problem.

Help is really appreciated, because this issue is causing us a
lot of problems.

Thanks guys.