IP4 only on one interface card and IP6 only on another???

I have a little problem! I "need" to combine two databases currently on separate machines into one. Each is on separate subnets. One is an IP4, the other is IP6. However for security (and bandwidth loading as the IP4 segment is still on 10meg till june), I need to ensure what is on one subnet, DOESN'T go to the second. Has anyone already attacked this problem and can give me some do's and don't's pointers or shortcuts. I'm already re-reading my networking notes to see how they were setup early last year, then i'll attack the net and SuSE's help and how-to systems for clues. thanks in advance for any guidance scsijon