How prohibitive is Glory to Rome's complexity to new players?
Disclaimer: Glory to Rome is one of my all time favorite games, I may be biased in encouraging others to try it :)

I would say the chief source of complexity and challenge to people new to this type of game is multiple roles for the same card.  Players who are able to reconcile in Race for the Galaxy that the same physical piece of cardboard can be a planet or development, a good, or a payment for a cost, should not be too challenged by this.  The same physical piece of cardboard filling different purposes in different contexts is fundamental to Glory To Rome.

The overall complexity of the game rules is mitigated by the fact that for each turn a player leads a role and everyone else must follow.  So in any given turn everyone has basically the same reduced set of options available to them, and you all learn together.

My opinion, anyone who's not intimidated by the insane information density of Race for the Galaxy cards, and has wrapped their head around the context-sensitivity, will have no issues with Glory to Rome.  (and can master its functionality, if not strategy, in one playthrough.)

My only negative comment might be it may become more luck based with only two players who have mastered the fundamentals, similar to Race with 2 players where it is close to being two solitaire games.  (That is speculative based on the observation that in larger games players can get boosted back in by people who needed the same lead they did, or held back by conspiring social forces.)