A simple cable "Y" splitter will allow two guitars to work on one amp, if one is using a "clean" sound [no deliberate distortion] and can live with the fact that each guitar's volume control will have some effect on the volume of both guitars (the volume control on each guitar works by both restricting the flow of sound to from the pickups to the cable, and swallowing up some of the sound that would make it to the cable; if a volume knob on either guitar is turned all the way down, it will swallow up all the sound from both guitars, but if e.g. one is set to full volume and the other one to half volume, the guitars will probably play somewhere around 80% and 48% of their normal full volume). Provided that neither guitar needs to set below about 10% of normal volume, it should be possible to achieve good volume settings by setting both guitars to about 80%, adjusting the amp so that the guitar which should be louder is amplified to the right level, adjusting the guitar which should be quieter to its level, and then increasing the volume on the louder guitar to compensate.
My biggest complaint with trying to use one amp for two guitars is that, as a performer, it's helpful to hear the sound of my instrument coming from its own speaker. If both guitars come from the same speaker, then when I hear a wrong note it's harder to know if it's my mistake (and I need to fix it), or someone else's mistake.