Have one flow connect or flow, so to speak, into another

[RafaelBorba]

Maybe (about Engraving Options):

But, about playback:

2 flows in 1

There is no other way. As two different flows could have different instruments attributed to them, it makes the process of stitching together way more complicated that Split stem (which is available). So copy paste is the solution. Make sure both flows have the same instruments ticked in. Then It’s quite easy to do in galley view, if you select all in the second flow, copy, select the last barline of the first flow and paste. You might need to delete useless time signatures and key signatures.