I have a friend who hiring one bun and one yan. No problem at all since yan cooks and cleans and bun take care of her girl. So you need to separate good of their job.
Forgot to mention, I also have 2 maids. Even they are relative but sometimes they will complain (to my mother) that another one is lazy, always on the phone etc.
i'm sure its okay as long as two can communicate.. but be sure to let them know their own responsibility.. to avoid the other to depend the other one to do her job... and become lazy.....
but of course if the "yan yan" can handle two kids, can "fire" the bun bun.. since you said she's not that good....