Hi,
Without the workflow attached it's difficult to see exactly what you're after here.
However, I am assuming that all you'll want to do is connect all of the Summarize tools to one Union tool (as long as the data structure is consistent, i.e., [Account No] and [Sum] fields), line up the fields as required, and then connect the Union tool to one Output.
Let me know if this isn't the case and I can help further.
Cheers