Dear Martha,
I think using Level B with both children will be the best way to go. Yes, you will need to get a separate workbook for each child and also two abacuses so each will have his own to work with together. By teaching them both with Level B you can be sure they have a firm foundation and no gaps. I would also recommend you play lots of math games to be sure they have fluency in their basic facts. If you have any more questions, please ask. We want you to be happy with your purchase.
Nancy