You can have multiple accounts on Elite, thats fine. But the subscription is a different thing. I highly doubt one subscription would work over multiple accounts. For example buy it for the 360 and get it on the PC and PS3 accounts you've added.
The codes aren't mutually usable as one systems codes are longer then the others. And if you link your names it doesn't actually mesh your accounts, it just puts them both in the top corner so you can easily switch between them.