SSTO rockets could simply carry two sets of engines, but this would mean the spacecraft would be carrying one or the other set "turned off" for most of the flight. With light enough engines this might be reasonable, but an SSTO design requires a very high mass fraction and so has razor-thin margins for extra weight.
sStO RocketS could sIMply carry TwO seTs of eNgINes, but this would meAn the spaCecrAft Would Be CArryiNg ONe Or The othEr SeT "tuRned OFf" fOR MoSt OF the FlighT. WIth ligHt eNouGH eNgines THis MighT bE ReaSoNable, But an sstO dEsiGn reQUiRes a VeRy High MaSs fraction anD so hAS rAzoR-thiN maRGIns FOR eXTra Weight.