[bread_baker]

I've taught multi-level classes before, with many more students than you have. I would try to find activities that you can run simultaneously, 20 to 30 minutes long each, one for each girl. You'll need activities at different levels. You can help each individually (alternating) during that time. I would emphasize bringing up the lower girl's skills. It seems that the younger girl can't think in English yet, and that is a big problem for her.
If it was up to me, I'd maybe try this:
First 20 minutes of class: each girl has a different activity
Next 20-25 minutes of class: each girl has a new and different activity
The last 15-20 minutes would be for a game that they play together.
I think the younger girl's education in English has given her a lot of reading but little listening or speaking. I would assume she has not learned any phonics. I would definitely teach her some phonics. I would even do some dictation with her. Ask if it's OK to give the girls a little homework each week. If you get that approved, I'd give the higher girl vocabulary to learn or practice, and I'd give the lower girl phonics and words to practice saying. I hope my suggestions will work for you.