If you only got to level 20, you might not need a whole new workbook for the younger child either. We're currently on lesson 15 of level A, and I think we've only used 2 worksheets so far... tally marks... There are a few coming up fairly soon that are just practicing writing numbers.

You could easily replicate those exercises by making up something similar on your own.

Anyway, depending on your kids, you probably will want to go with level B now for the older one, it moves a little quicker but it still starts at the beginning, so he won't have missed anything by skipping most of A!

Or, if you feel he's "weak" in math, or if you think your two kids are pretty even in math, you could try doing them both together in either level A or level B (if you think the younger one is ready for it). In that case you would indeed need an extra workbook for the other kid. ;)