If you are using the SWMM5 engine you'll find a field "Barrels" in the "Couduits" editor.
If you are using the MOUSE engine, I don't think there is a similar way of modelling more than one pipe. You'll probably have to draw one pipe beside the other. The good thing is that you can offset them visually by using vertices, even if as they have the same from node and to node.