Mixing two different scents may not work, especially when you put one so close to your nose. Unfortunately, very few scents have associated aftershave balms. Alcohol aftershaves disappear in minutes, leaving the nose free to smell the cologne, so there's no problem there. I would go for a scent free balm.
If you really must, you should focus on the base note, which is the one you'll be smelling most of the times. Beware, as usual, that listed notes are often pure fantasy, so you should check with your nose.