The weak hive can't build up with 2 frames of bees. It just takes too long. She can only lay what eggs her bees can take care of.
Your on the right track by giving her some brood. That will boost her nurse population and she can lay more eggs. That brood will hatch in less than 9 days. About that time I'd give her another if she needed it. A frame of brood will make two frames of bees or more.
Unless they had a flow on I'd feed the strong hive and swap out frames with the weak one. A frame or two every week or so. It wouldn't have to be capped, open necter will be fine, possibly preferred.
The weak hive needs all it's bees to raise brood their not going to be able to spare any for foraging.
I'd just shake the bees off when I exchanged the frames.
If it got too late in the year and it didn't look like they'd make it I'd combine then. Good luck, Woody