It is quite easy. First you need get the structure for each pathway,

For example, for the first pathway

A - B
B - C
A - C

And then, for the second pathway

A - F
E - M
T - H
M- H

Then you can merge these two pathway and import them to Cytoscape, then you will get the merge pathway.

Our regulatory network browser might offer some guidance on how to do this via a d3.js-based website. Otherwise, you could try rendering a couple networks in Cytoscape and pull exported figures together in Adobe Illustrator or Inkscape.