From my experience it seems quite normal. Your success will depend on how long she leaves the nest for, and as she is a first timer - she may be leaving the nest for too long. If two hens can lay in the same nest while she is away i am assuming that she left the eggs for over an hour? if this is the case then this may well result in a non-hatch. My broody never leaves the nest for more than 30 minutes, but she is experienced in hatching eggs so whatever she decides to do, its fine by me. It may well be that she has just become broody and that her desire to brood will get stronger in a few days (i.e. she will not leave the nest for extended periods of time). If this does happen, i would suggest removing the older eggs and starting with new ones (you can save some starting from today, and put them under her later).
Then nesting box is fine, and sometimes if you decide to move a brooding hen she will stop sitting on the eggs. I have nest boxes around the same height off the ground, and my broody has been sitting for 9 days now. When the chicks hatch, i will move them to a mini-coop on the ground. I have done this successfully on 3 occasions this year already.
Good luck!
CT
She is on day 2 1/2 of being broody. She was off the nest a little this morning to eat and drink. I marked her eggs while she was doing that and she's been on the nest all day. I'm wondering if they'll hatch. I've read posts of folks saying that their girls were off the nest for hours and the eggs cooled at points and still hatched. I guess I'll candle the eggs in a few more days and see if they are developing or not. Then change them for fresh ones if needed. This is a new thing for both of us.