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Can someone explain this concept to me? (site-to-site Tunnel)

I'd recently setup my first site to site ipsec tunnel but had a tough go at it until I got some help from the Cisco TAC.

I had initially configured the inside interface of this nomadic ASA(5505) to sit on the same inside subnet of the stationary ASA(5510). Since we have two devices, one of which is nomadic that need to talk, it seemed like a good idea. I was told by the tac technician that you can't build a site-to-site tunnel with the same network residing on each side. Sure enough, when I changed one side to a different subnet, it worked fine.

I'm assuming that these two ASAs are acting as layer 3 devices and thus seperate a site-to-site vpn tunnel in to 3 broadcast domains. Since that subnet already resides in an ASAs broadcast domain, it won't look for it on the other ASA??? Is this a true statement?

Is there a way to make it work the way I had originally intended? I've already worked around it, but i'd be interested in knowing.

I hope that all made sense.