If the amounts are small, and the relations between you are friendly, I'd think the simple solution would just be to have the amounts deposited in one bank account or the other, and then periodically -- monthly, once a year, whatever -- count up how much was deposited and the person who owns that account transfers half of it to the other. Amazon gives you detailed reports on how much they paid and for what books, so it wouldn't be hard to figure it out.
If the amounts get to be non-trivial -- whatever you consider non-trivial, hundreds of pounds maybe? -- you might want to have an agreement written up by a lawyer to keep everybody honest. Personally I wouldn't go paying hundreds to a lawyer to write the contract until our sales had hit hundreds of dollars. :-)
I'm in a vaguely similar situation myself. My step-father is writing a book and, knowing I had published several, asked me to help him with the mechanics. I've put his book on my Amazon account, and I figured I would just forward any money to him as it came in.
Afterthought Months Later
You might think, Hey, why doesn't Amazon just provide a feature to split the money between accounts? As far as I know they don't. But I'm a software developer myself and I once worked on a computer system for paying royalties in a different context, and it was amazing how fast it got very complicated. First off we had to consider unequal splits, like maybe there are three people and it's divided 50/25/25. Then what if someone dies and leaves the rights to his heirs? Now you have to divide one of those shares by whatever proportion among the heirs, so maybe its 50/25/12.5/12.5. Are the proportions constant or are their tiers, like if one person put cash up front for expenses, you might say the first $1000 all go to him and only royalties above $1000 are split. If there are multiple books on the account, different books might have different splits. The software I worked on got very complicated. Amazon might well have decided they just don't want to get into it. If you want to split the royalties, you deal with it.