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Aug 6, 2017 at 20:10 comment added Mark Grant A similar question was asked here: mathoverflow.net/questions/27702/… Note that the homotopy $f$ already gives a null-bordism of $A\cup B$, so you just have to decide if $f$ can be made an immersion (for which there is the Smale-Hirsch theory) and then an embedding (for which there are variants of the Whitney trick, as mentioned in Oscar's answer, but these will be highly sensitive to the dimension/codimension).
Aug 6, 2017 at 18:08 history edited DanielHarlow CC BY-SA 3.0
changed continuous to $C^1$ since implicitly the question used immersions.
Aug 6, 2017 at 18:01 history edited DanielHarlow CC BY-SA 3.0
modified to include a point form the comments.
Aug 6, 2017 at 17:45 comment added DanielHarlow You mean because the derivative matrix of $f$ might not have full rank? Or did you have some other obstruction in mind?
Aug 6, 2017 at 17:42 history edited DanielHarlow CC BY-SA 3.0
added 7 characters in body
Aug 6, 2017 at 17:42 comment added John Pardon The version of the question for immersions does not have an obvious answer as you suggest it does. You can't always make a map an immersion using general position arguments.
Aug 6, 2017 at 17:33 history edited DanielHarlow CC BY-SA 3.0
switched "homologous" to "cobordant", which is more accurate.
Aug 6, 2017 at 17:26 history asked DanielHarlow CC BY-SA 3.0