You can't find anything barring this because there isn't: you can stack these properties, and really, most if not all other properties. Weapon properties are expensive: what makes you think you shouldn't be able to get your money's worth?
The command activation isn't really that important: once you give the command, the energy damage stays until you turn it off. So even if you activate the damage separately, you still have both types of damage, for a total of +2d6 damage of various energy types.
I have never played a game where the activation of these properties ever came into play: they have always just been assumed to be there when you want them. I can't really think of a good reason to get more not-picky about it.
Also, note that for a bow, there is also the option of using magic arrows. You can combine a +1 shocking bow with +1 flaming arrows to get an attack that basically counts as +1 flaming shocking.