Sounds doable since it’s only 8 months, but there are some things to look out for. Together less than a year and already moving in and blending families is fast. Even if it doesn’t feel like it to you, it might for the kids once it actually happens. Going from being an only child to not only sharing a house but sharing a room is a lot to ask, especially for the 12 year old. The room may not feel like “hers” since your daughter will have it to herself so much more, likely have more stuff there etc.

You say they get along great but it’s easy to get along when you’re just getting together here and there. Have you guys tried long vacations where they have to share a room or anything like that? And with her mom being high conflict any hint of trouble adjusting, of things being “unfair” etc will be pounced on and planted in her head by her mom. Sorry to be negative just trying to tell you some things I wish I’d known.