tankless electric hot water heater
That would be paralleling. You can't do that.
Certainly, 120/240V split-phase can be confusing to the novice.  But when an amperage is stated, such as 60A, that always means exactly what it says -- you never halve or double it because of the split-phase.
Being a 240V breaker/circuit merely means it has 2 poles instead of 1.
&quot;120V per pole&quot; is actually how you think of it, but again, never multiply/divide.
So for instance it would have been acceptable to use two 120V 60A breakers and handle-tie them... (of course 60A 120V breakers are not made, but if they were; yes).
The only possible way to make two 2-pole 30A breakers work would be a cobbled-together version of paralleling... the paralleling rules are quite narrow and don't allow anything like that, for a variety of reasons.  Paralleling equipment is really special.