Well, firstly, the Jet engine actually burns more fuel at idle at low altitudes like when it's coming n for landing, than it does when it's at 30, 000 feet with full throttle up! That's how efficient they are! Now, with me saying this, we can conclude, that 2 smaller ones means probably more weight on the wings, but also if one jet fails you have other ones. So, it depends on the airplane size, if the wings can hold 'er as I like to say, then it probably better to have 2.
I hope this helps, please rate my answer.