Make One Full Power XBox One Adapter from Two Underpowered Adapters
Putting two voltage sources in parallel is usually a Bad Ideatm, unless they are expressly designed for such. If these supplies are regulated (and they may or may not be), then each might be slightly different voltages, even though they both say "12V". They can't be exactly the same, due to manufacturing variation.

Consequently, if you put two in parallel, one may end up providing all the current and overheat. Without knowing specifically what power supplies you have, we can't know for sure if they will start a fire, self-destruct, simply fail to work, damage your XBox, or work fine.

But why bother? It is not difficult to find a 12V, 17.9A or 5, 1.0A power supply, if you look outside the "XBox" microcosm. Why not just buy the right power supply and put the right connector on it?