I can think of a few twins which burn less on two engines than my 18 gallon an hour IFR SEP.
The DA42 does (11GPH combined at 140kt IAS) but when the engine packs up (which they do fairly often) you are grounded for an awfully long time ....
What kind of an IFR SEP do you have that burns 18GPH? I know the Cirrus SR22 and the Lancairs do, to achieve the sales brochure TAS figures, but that is at 75-85% power which is an awful lot; one should not cruise at such power settings because the motor is unlikely to make TBO.
I agree with Englishal re 30GPH+. This gets pretty poor value for money. Mind you, one can take the legitimate view that you bought the plane so cheap that blowing money away on fuel is still worth doing. Such planes are great for the "VFR in all weather" kind of flying which a lot of the owners do in them, to avoid the Eurocontrol route charges. An Aztec can fly with inches of ice on it.