It happens sometimes and sometimes it works out okay, but usually it is not a good idea to let two broody hens share the same nest because there is too much competition for the eggs, even if it does not appear that way to you. When a broody hen sits on her eggs, she monitors them and keeps them at the same humidity and temperature and turns them as needed. If they've been sitting on the same clutch for several days, just let them go, but the next time this happens, I would immediately take one hen and separate her from the other in her own nest box. Make sure that it is dark and closed in so that she cannot escape. Put her in it at night and leave her in there, with a secure container of water, for the following day. If the weather is hot, make sure that the nest box is in a cool place. The next day you can partly open the nest box a couple of hours before the other hens go to roost. If she goes back to her old nest, wait till it is dark and then put her back on her nest. Do not leave the nest box too long because you don't want the eggs to get too cool. Then close her in the nest box again. Once you are sure she is going to stick to the new nest, leave the box partly open so she can come and go as she pleases.
Thanks for the advice.
