I have two SliTaz virtual machines configured exactly as described here:
http://forum.slitaz.org/topic/internet-connection-sharing/page/2#post-1335 , which makes things simpler to explain ;)
Don't think, their being virtual changes anything. Let's call them 'server' (the one w/ two adapters) and 'client'.
So, I set up IP forwarding on the server just as described in the post above and it kind of works, but something's still missing from the setup, b/c (I'm talking about the client machine here!) I can only ping internet hosts by IP address, but can't use their names. On the server everything works as intended - I mean, I can browse the web, ping the client etc. etc.