I love the dual band mode, that's no secret. Keeping one ear on a dedicated simplex freq. while scanning others is great. I've never used the cross band repeat mode though, other than one time when I was trying to figure it out.
I don't see any added value in the 8900 personally, but if a good deal on a 8800 were to pop up I'd seriously consider buying a 3rd one, would put it in the XYL's car.