If your new broody isn't at least 18 days into a solid set, I wouldn't try to give them to the new one. Rarely, and I mean ONCE, I got lucky with an experienced hen at 2 weeks, but keep in mind I'm here all the time and I keep my broodies in crates until either the hatch is done, or if I did surrogate chicks, until SHE is ready to get up. With my last batch I had to hand-raise the chicks for a week because their hen wasn't ready - I'd guesstimated how long she'd been broody (always write it down!!) and I'd miscalculated.
If once you get the new chicks, it's running close on the new broody, set the chicks up with the MHP instead of a heat lamp. It teaches them to find a nice warm dark place (like under a hen) and is the easiest transition- and keep them until her day 18, then give it a shot.
I've had 1 with older chicks already take much younger chicks - and happily the older chicks watched out for the littler ones after momma hen was "done."
I've had 1 with young chicks take much older chicks - and because I'd done the MHP with them (momma heating pad cave) that simulates going under a hen, I think that helped them adjust to an actual hen. I believe they were 3 weeks old- their intended mom didn't accept them and decided she was done. They loved having a momma hen.
What I did was put the want-to-be-accepted chicks in a separate container (crate, bathtub, whatever is easiest)nearby to the momma hen with existing chicks- and if momma starts talking to the other babies, then I try slipping them in overnight.
But again, I'm here -- and there are no big spaces of time where I'm out of earshot (and I keep them on a baby monitor until all is 100% well). So there's a chance- but frankly momma #1 might be wrapping up her chick rearing- and at (edited to 4 weeks, put 6 in error) 4 weeks old for the original batch, it might also be too big of a size issue to be a safe situation.