The answer is:
Can...!
If the two flavours are similar, you'll end up with similar flavour with different PG/VG ratio, that's all...
If nic strength is different, see bro Karl's post above...
Nothing else will happen, there will still be vapor... Only thinner or thicker vapour as result...
I sometimes mix n match my juices, but from different flavours, especially when i'm feeling bored in office... Most of the time I got weird tastes when mixing "incompatible" flavours...
