I should add that those two Yog-Sothoth faction resources don't need to be available (i.e. undrained) for steadfast - they just need to be there. But as per usual, you do need an undrained and suitably resourced domain to pay the '1' cost.
For example, it wouldn't matter if Domain B was already drained - it's 1x Yog-Sothoth resource still counts, along with Domain A's undrained 1x Yog-Sothoth resource, to total two for steadfast - and then you could still drain Domain A to pay the 1 Yog-Sothoth cost.
The purpose of steadfast is to ensure that these (predominantly low cost) cards are only used 'in faction', with a deck that has a sufficient representation of cards from that faction.
Edited by jasonconlon, 29 December 2013 - 05:16 AM.