My vote... don't do it.
You really should have different path codes so that files are NOT shared between foundations (i.e. Central Objects, Control Files, etc.). This precludes you from having to worry about any changes as to how data gets written by different foundations. Also, if you plan on building packages using both foundations, you'd be writting to the same folders on the deployment server and possibly deploying SP22 packages to SP21 clients and vice versa.
The only way I could see it all working is by using different machines altogether.