How to close two loops at once

I want to append a column to a table row by row, because the pivot column will look different in each row. I have therefore started two Chunk Loops. But after the Column Appender I can only close one loop. I thought this is exactly what the node Loop End (Column Appender) is for.

But I get the error message: Can’t merge FlowVariable stacks! (likly a loop problem). What can I do? How do I close two loops at once?