I have to be fair and say he would probably rather have them overnight than have to do all the driving,
Yes, this is what would solve him going to and from your house twice a day, on consecutive days at times that affect your work
but they don't want to stay as he's never made them feel like it's their house.
He's not been there that long, and it's his gf's house, so it makes sense that they don't see it as their house... Why would they? It's where their dad has just moved too, and a strange place to them. Yes, he needs to make them feel welcome, but you aren't helping yourself with the "it's all down to him not me, and I won't make them" approach. It would help to explain that no "new house" will feel like theirs. No, they don't have a bedroom, but that doesn't mean they aren't wanted, the house was like that before the gf even met their dad, so now he lives there too, it's something you all need to work round together
They also only have 2 bedrooms, and at 11 and 15 of opposite sexes, sharing a room isn't an option
Ok, so one child has the spare room. The other can have their room. They can put a bed in the living room for the few days the DC are there. Or a divider in the spare room. You can all make a reason not too, or you can find a solution.