Will you be spacing it out a week apart like Boris suggested?
DH is a stickler for the rules and watched the breifing so I think he is going to want to do this, but he will also want to see his mum (or the kids to) next weekend and then suggest mine the week after. His rational for this is fair. The kids normally see his mum every week and mine only once every 4-6 weeks as his mum lives 10 mins away and is retired and mine lives an hour away and still works full time.
However, I really want to see my mum and she has been really sensible, and understanding unlike MIL who is just desperate to see the kids and happy to break the rules to do it.
I don't feel the need to choose. I am happy to see his mum on the Friday morning and my mum on the Sunday afternoon etc. Will you be staggering it or just cracking on?