using just one pole of a 2/pole breaker?

So I messed up, nothing new ...; I had ordered a 30AMP transfer switch (Midnite Solar) from NAWS, and only noticed that it was a 240V after it arrived. Trouble is I'm using it to switch between the generator (120V and a 120V inverter), so I'm wondering if I could just use one side of each breaker. I would normally just replace the breakers, the only reason I ask is that it they are installed to fit a metal "slider", and changing to single pole breakers would likely create a bit of a loose fit.
I'm hoping for a simple solution to a stupid mistake, thanks ...
HB