Re: 240V electric furnance - what is Dual Supply Circuit Vs. Single Supply
Use the dual breaker method -- whoever wrote the manual needs a Code-slap with 424.22(B), as the single breaker method doesn't comply with it.


  (B) Resistance Elements. Resistance-type heating elements in electric space-heating equipment shall be protected at not more than 60 amperes. Equipment rated more than 48 amperes and employing such elements shall have
  the heating elements subdivided, and each subdivided load shall not exceed 48 amperes. Where a subdivided load is less than 48 amperes, the rating of the supplementary over-current protective device shall comply with 424.3(B).


If you're short slots -- use a double-stuff (quadruplex) breaker.