As long as you are collecting daily, your eggs should be fine. I too like to collect twice a day if I know a broody is sitting on it. Eggs have to be heated for 24 hours before they begin to grow the embryo.
Yes, 2 broodies often try to hog a broody box together. If you were actually hatching chicks, I generally encourage to separate the hens as *most* hens cause each other problems when they try to brood together.
As to how long they have this behavior, that totally depends on their genetics and hormones. Many commercial breeds only flirt with brooding and are quickly over. Some of the more heritage type birds take brooding to heart. Silkies are diehards and will sit prolonged periods of time.
How easy to break them depends again on the hen. Some only need to be removed from the nesting box a couple of times a day and given treats. Others have to be placed in a "broody buster" cage with food, water, and air flow to break them.
I like to use my broody hens, so I don't break a brood because I believe it gives them bad brooding habits. Most of my hens, if I don't use them, break within 4 weeks.
I'd try by blocking them out of the nest and removing them from the coop a few days before going to something like a broody buster cage (edited) since you never plan to use them.
LofMc