Yay! Congratulations! Silkies make excellent mothers, and go broody white often which is fun.
For both your mamma's, if possible try to find a way to separate them from the others. You can do this by using a pet, or baby gate, thats how I separate all of my broody's. Its best to do this because other hens will try to lay under her, or could damage the eggs while doing so. In most cases the broody occupy's the best nest box, and other hens will get irritated that she's not sharing.

As long as you leave them under momma, it doesn't matter which day lockdown is, she'll know and do her best for each individual egg.
I read on here before that broody's can sense whats going on in each egg, and adjust so that egg will thrive. Thats why shipped eggs often have a better survival rate under broody's thats in incubators. Congrats and keep us posted!