Vortex Sound Research

Hi, I have an apogee duet 2 which only has 2 inputs. At first it didn't bother me because it wasn't a hassle just switching out the audio cables for whatever hardware synth I would use but now my collection has outgrown that solution. Do I have to get another interface to be able to have more inputs for my synths or is there a more cost effective solution? If it has to be an interface, are there any specific budget ones that are good? Hope to get some feedback on this. Thanks.