haha, the same case w/ me. 2 bun bun with 2 kids. i had the same problem before and now settled. so let me share my experience for your reference.
1) old maid take care big kid (as before)
2) new maid take care new baby
3) they take turns to do housework: separate the housework by 'kitchen' & 'outside kitchen'. kitchen include buying food, cooking, managing fridge, washing dishes; laundary (the washing machine in kitchen). They take turns by week.
4) at first i would like the new maid to take care of the new baby every night, but it is really no good - too tired, you know babies can wake up many times at night. so, they at first suggest take turns by week; but they find it too tired. so now they take turns by day.
Luckily, my two maids are quite co-operative, so sometimes they help each other and there may not very clear of duties. so:
1) one maid responsible for one girl (i can blame for).
2) set a form for recording their 'night-shift', so can calcuate and make it fair.