okey did a test
my setup: AMD Phenom II x6 1600T @ 4ghz
24gb 1600mhz ram
okey, so, i had 3 VM's. and i gave 4gb to to linux and 8gb to windows ram to each of em.
so: 1x windows VM
2x CentOS VM
so that puts it at 16gb usage, leaves 8gb more, ram isnt the issue at all
however, when you are trying to run the servers, you can run about 2 realms for each one
2x WOI and 2x BOI, this would put my cpu at a total of around 50% useage, estimated max is around 3 realms of each server i guess, i did try with 4 of each but this would put my cpu at 90~100% usage, possible but if the server gets any real load you run out of CPU space, so thats not recommended.
so, if you are really serious about this, and you want to run both servers on 1 server i recommend atleast 8 cores (if you want to run all 8 realms for both servers i would go and look for a server with 16 cores at least)
every gameserver uses around 500~600 mb ram, so to be safe i recommend atleast 16gb, but again if you want to run 2 servers i'd go safe and just take 24 or 32, afther all your VM's need RAM too, the MySQL and MSSQL server both need RAM.
be warned though, i wouldnt recommend any of this to begin with. if you want 2 servers i recommend you get 2 physical servers, each running their own part. not running everything on 1 server.
(oh and i recommend a good SSD with 128gb and high read/write speeds if you really want to run 8 realms of both servers on 1 physical server.)
i wish you good luck with it, but its doubtfull its going to be any stable.