I can easily create a document with two connected flows on the body pages. For example, I can create a new Portrait document and add an autoconnected flow tagged B to its Right master page. Once I've done so, I can go back to the body page and start to put content into either or both flows. As expected, FM adds new pages as needed. Flow A on p. 1 connects to Flow A on p. 2; Flow B on p. 1 connects to Flow B on p. 2; succeeding pages. The content on one flow may fit in fewer pages than that of the other. Also as expected, pages may become empty if I delete content and when I save the document, FM discards the empty pages.
Suppose, however, that I have an existing multi-page document with Flow A but not Flow B. If I import master pages from the new document with both flows, I get both flows on every page. However, Flow B on one page is not connected to Flow B on the next page. There are several ways to see that they are not connected:
1. Text symbols display an end-of-flow symbol in Flow B on every page.
2. I can insert content into Flow B on a page other than page 1.
3. If I add more content than will fit on one page into Flow B on page 1, FM adds additional pages between pages 1 and the original page 2 to hold the overflow.
If I copy all the content in the original one-flow document to the clipboard, I can paste it into an empty two-flow document. The result is a two-flow document in which Flow A has the original content and Flow B is empty but connects page to page. I can use Save As to replace the original file with this two-flow version.
Is there a way to modify the original document and obtain the same result? Somehow connect the multiple B flows that importing master pages creates?h
Suppose, however, that I have an existing multi-page document with Flow A but not Flow B. If I import master pages from the new document with both flows, I get both flows on every page. However, Flow B on one page is not connected to Flow B on the next page.
FM 2017: I can confirm this behaviour:
By connecting two text frames, you only get a single Flow B:
You are correct. Once created, the flows can be connected. The effort involved to do so manually, however, is not practical for a large number of pages. Copying the content of each original document into a new one is easier. For a large number of documents, either approach needs to be automated either with a script or a plugin.