Okay.. well I thought I had tried it - BUT
Indeed if you have 2 Sub-Accounts from the SAME provider - each sub-account will need a SEPARATE ITSP profile configured (even though they WILL BE IDENTICAL) and a separate SPx service .. I tried it and it worked a treat!

So another tidbit I learned about vitelity from this experience - if you have a PBX (DID routing PBX style) then you can use the 2 proxies (typically sipXX.vitelity.net and outbound.vitelity.net) HOWEVER if your DID routing style is ATA/IP Phone then you should only use the sipXX.vitelity.net proxy server and don't even use the outbound.proxy.net.