comments:
It's generally best form to wire your transfer panel as intended (so L14-30p inlet for example) and then to use a bridging (aka "dog bone") adapter like this ( https://www.amazon.com/CircleCord-Generator-Transfer-Adapter-L14-30R/dp/B08XNTPX98/ ). That said if you are hard wiring you can indeed just bridge your input legs to form a single leg. (50a inlet would be ss2-50p instead) Caution: If you have any multiwire branch circuits (circuits that start out as a 240v wire going out of your main panel before breaking into 120v elsewhere) you should not power them in this manner, you can have unintendedly high neutral currents (15a on l1 and l2 would normally cancel out the N load, but if all the same phase you would have 30a coming back on the N)