I would not do it, I would run both forums in 4.2.2 not vB5, there are to many issues with it still.
Once you purchase a new license, you have access to the vB4 an vB3 series, no need to use vB5.
I know, but anyway, one day I will need to upgrade to vB 5. Is better to face those problems now, than other day in the future. For example I have doubts about how to configure my core config.php for xcache, this will not change in the future if I not solve it now.
On the other hand I was attempting to run vB 4.2.2 on my new server with PHP 5.4 and I got some warning errors, may be because not all incompatibilities between vB 2.2.4 and PHP 5.4 are solved yet.