I've tried this in the past, and it was really difficult. I dare to say that it's almost impossible to give value to 2 different skill levels at the same time.
You are 3 options here:
- Tailor the lessons to the lower level students
- Tailor the lessons to the upper level students
- Divide the class activities into the 2 different levels
All 3 options wont be ideal to any of the students. You just have to make best of the situation you are placed in.
At the beginning you can mix up and teach different greetings such as:
- What's your mother's name?
- Where does your grandmother live?
- When were you born?
- What's your favourite food?
Once again, with these new greeting presented, both groups will be learning from scratch, so it's fine to combine the groups.
I would definitely assign homework and in-class activities to each group to suit their particular level. Try splitting up the groups so that they are apart from each other by a few metres, that way they wont get distracted when you are explaining to the different level.
At the end of the class, bring them all together and do a vocuabulary building exercise. Try to teach them new vocab that neither group knows. That way they both start off on the same level playing field.
Good luck and hang in there!