This morning, the security lines before terminal A were very long, but in terminal B they were pretty short. So I asked one of the employees at the beginning of the B security line, whether I could freely walk from Schengen B to Schengen A and he said no. The FRA Airport app said it was possible through a tunnel and this thread seems to concur with the FRA Airport app.

I didn't want to risk it and miss my flight, so I went back to the overflow queue between terminals A and B. But for future travels, can somebody confirm, that you can go through B security and then walk over to A without another security check?