If the rest of the house is one-pipe and not banging, why does he think two are needed for your new radiator?

You might want to ask for another opinion. It's really difficult to answer your question here.

Kalining always has some good advice about steam systems.

It is true that base board radiation and steam are a combination for banging pipes...your existing cast iron radiator sounds way over sized for a bathroom,usually 8ft. X 5ft.,a 4section recessed radiator would do just fine,and, save you some floor space..of course there would be some repiping involved .....

When remodeling my old farmhouse 1 pipe steam, I went
to a metal salvage yard. I measured the old radiator
pipe input. You just turn off the floor valve and unscrew
the Union joint nut and the radiator come off. Also
measure the distance from the floor to the
center of the union opening.
At the yard they had big, small, Victorian, modern radiators
for about $100 - you can talk them down to $75.

Buy the one you like (might prefer to have wife
make the choice). get it home clean it up screw
the union joint onto the floor valve level it
with shims so it slopes a little to the valve.
Open floor valve, install old air valve on the
other side. And you are done.

Keep the old radiator because you will find out
later that your wife complains how cold the
old bathroom is.