Copy a flow into another flow? Easiest way

Is there a magic way to copy a flow into another flow?
A brief sketch here:
I have flow1 (piano) and flow 2 (piano).
Starting with an empty flow3 (a score for two pianos)
Copy flow1 to flow3 piano1 starting at bar 5
Copy flow2 to flow3 piano2 starting at bar 7
Copy flow1 to flow3 piano2 starting at bar 23

I understand this can be done “by hand” copying and pasting
moving back and forth. But is there a magic easy way to do it?
Or, what is the easiest way to do it?
Any suggestions might be useful. Is there something in the
“Lua” world (I haven’t been there yet) ?

(I’m working on a piece where I would like to do these kind of
operations a lot.)

You could save yourself some of the work by starting flow 3 as a duplicate of flow 1, say.

Absolutely. Good idéa, I sometimes use.
No my question is about how to do a lot of these operations
(maybe even hundreds) and every way to do it easier will save a lot of time.

Thank you for your reply.
Reine