2 inputs, 1 output

My amp only has one input, but I want to be able to plug in two guitars at the same time. A passive ABY would do it, but as I understand it there can be issues with volume and tone suck. Do I just run the two inputs into a summing opamp, or would that not be a good idea/is there a better way? Also, would using the volume knobs on the guitars be enough, or should I include a 'blend' pot?