Is it safe to use two single-pole breakers for a 240 V circuit?
You are correct.  The issues at hand are common maintenance shutoff (to solve the problem of which you speak) and common trip (to entirely remove power from a device with a problem).

If the circuit serves a device that uses only 240V and does not supply neutral, then common maintenance shutoff will suffice.  Go to an electrical supply house that deals in your panel/breaker type, and obtain a proper, listed handle-tie.


Proper, listed handle ties can be hard to find, especially on a Sunday.  An adequate substitute for a handle-tie is a 2-pole breaker.

If the circuit serves a 120/240V device (which takes 2 hots and neutral), then common trip is mandatory.  Go to any hardware store and obtain a 2-pole breaker appropriate for your panel.

While you're at it, make sure they didn't slip you an alien breaker: a breaker not UL-listed or UL-classified for that panel.  For instance if they slipped Homeline breakers into your Siemens panel, a Siemens handle-tie won't fit those breakers, now is the time to fix that.
Also, while you're labeling, put &quot;GFCI protected&quot; on any outlet fed off that GFCI. That's a code requirement most people overlook, causing home inspectors to &quot;write them up&quot;.
Why doesn't a handle-tie work for common trip? Shouldn't the tie transfer the trip motion to the other breaker(s)?  No.  Breakers are required to &quot;trip free&quot;, meaning trip even if the handle is held in the &quot;on&quot; position.  (E.g. by a breaker lock meant to prevent tampering).  Common trip is provided by a special internal mechanism.