I have conducted one focus group and one interview where the same questions were asked. Could I analyse both data sets together using thematic analysis?
Firstly, the purpose of conducting a focus group discussion is different than conducting an interview. The data collected in both these approaches for the same set of questions will be different.
You might use the same coding technique to analyze the data collected through focus groups and one-on-one interviews. However, the data from the focus groups will have perspectives/experiences of different participants as against just one participant in an interview. You might want to describe different aspects from focus groups about a specific group and an individual's story from an interview.
I myself do not see any difference in the purposes for conducting focus groups or individual interviews, so I think it there would be no issue in conducting Thematic Analysis on each of them.
But this is a rather small qualitative data set, which might fail to reach "saturation."