[mesmark]

Well, you don't have to separate them just for worksheets. You can separate them for speaking exercises as well. Pair work and even group work can be done separately.

I'd probably teach to the lower level and then when we start practicing in a game or something work with the more advanced students to build onto what we are already doing.

Mixed levels are always a challenge, but reviewing the basics isn't bad for anyone. Having a solid firm understanding of those lower levels makes more complex sentences much easier to manage.

Good luck!