I'm guessing that you are talking about extensions cords? If so, how would you "combine" them?
Apparently I should be using 12 gauge wire for my electric mower as a smaller gauge causes damage. Currently have a 16 gauge. Can I combine two 16 gauge wires and thus meet the heavier carrying capacity of a 12 gauge?
If you are drawing a circuit on a piece of paper, it would probably look like it would work.
On the other hand, if it's an extension cord it's stranded and not solid core wire. And the amount of flexing it would have to do while you mow your lawn would tend to weaken some of the strands to the point they might break---which would change the effective current rating of the wires between the outlet and the mower.
That makes it more of a safety/engineering issue, and probably would be frowned on by a certified electrician. (I did rewire part of my house years ago, but only study that part of the National Electric Code that applied to the type of work I was doing (it all passed inspection)---and that gave me a certain feel for what is potentially dangerous rather than straightforward when you are working with alternating current.
That 12 gauge extension cord you need sounds cheaper and cheaper the more I think about it.