It's a minefield,as someone else said.
We're in a similar position with one house in England, and one in France, where we live. We have 4 children - one his 3 mine. We have a clause in our french contract to say that the house transfers to the survivor when one of us dies.
BUT when the survivor dies the house goes to only their children.
There is a way round it (partially) due to a recent EU law
It involves making a will that would be acceptable in the UK, but not previously in France, having it translated into french and lodge it with a french notaire. Not sure of the details but you should be able to find more on the internet.
I had a similar conversation as you did with husband and he was very hurt, said I didn't trust him. As I'm older than him, and likely to go first, he says he would sell up here and go back to the UK, as he could never cope with the bureaucracy, not speaking french. That would get round the problem, and he could leave a quarter to each.
Don't ask me what I've done - nowt yet but I've got a basic form.
We are halfheartedly trying to sell the french house now, but it's going to take ages.