Unfortunatly you would have to do a double nat policy in order to get site to site vpn using the same addressing space. Your only other option is to use two routers to create a GRE tunnel and then bridge the tunnel.
I'm assuming since your using 506's that these sites are realitivly small meaning it would be very easy to readdress on of them. That is probably the best recomendation for the long term.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...