When it seemed like an invasion was imminent, the Romulans were willing to join the fight to keep the Dominion on the other side of the galaxy.
Both are totally in character I think.
The Romulans almost always manipulate things from behind the scenes only directly involving themselves as a last resort.
The Dominion, who they acknowledge as a threat and the Federation, their cold warriors, fighting each other should if anything make them pleased. Both sides would be weakening each other and they stand in place to be able to pick up the pieces from such a destructive conflict.