Probable the least expensive approach would be to use a Daul-Wan router.
Connect each wan port to each network and config the printer on the multi-DMZ.
You would have to put the printer on the dmz of the router which would provide 1:1 NAT and would not conflict
with the NAT on each of the two networks.
Your networks would still be isolated and they could share the printer.
http://trendnet.com/products/proddetail.asp?prod=185_TW100-BRV324&cat=41I've never tried it, but you should be able to get it to work providing share access to the printer and Network isolation.
Your other option is to use an open-source firewall like pfsense and run that on another PC or embedded system.
Probably very much of an over kill.
The router doesn't know it's on 2 LANs or 2 WAN's, an IP is an IP.
With the 4 port router you could effectively share 4 printers between the networks.
It's a dirty trick of a solution but should work fine.... 99.9% sure it will work.
You could also share a SME server on that router the same way as sharing the printer.
Then have SME serve as a share internal email server.
I see no other cost effective way of providing shared access to the printer and Network isolation.
You could always return the router if it doesn't work, worst case.
Pretty sure it's the best solution that will work.
If you try this...please let us know your results.
HTH