There are various things you can do, and various things you can't, so you need to state EXACTLY what you want to do - not doing so could result in answers that could destroy your amplifier - so it's VERY IMPORTANT to be specific!.
This is still pretty vague!, I'm assuming you have a guitar amplifer with only one input?, and you want to feed two guitars in to it at the same time?.
You 'could' just make a 'Y' lead, but it will have loads of limitations. What you need is a small two channel mixer, a simple opamp and a few resistors is all it would take.
You could even try just two resistors, one from the centre output of each guitar, and joined together at the amplifier input. The screens of all the cables should be connected together as well. The value of the resistors depends on many factors, I would try 470K for a start, and see how that works.