Went to the local electric store and had some problems.

I wanted to wire a 220v plug on the welder cord and then make two pigtails to come off that, one for a nema 6-50 220 outlet, and one for 110v.
The guy at the electric store said that can't be done. He said I would need to physically change the plug from 220-110 each time I wanted to switch which I know is wrong.
How have you guys wired your welder to run off 220 and 110. What exact type of male/female plugs because he made it sound like this is impossible.

Thanks for any help!!