Activation: Details relating to rod use vary from item to item. Unless noted otherwise, you must be holding a rod to use its abilities. See the individual descriptions for specifics.
It varies, but for most rods you need to be holding it to activate it.
You have to be wielding them unless they say otherwise. You can't activate multiples at once because the rules don't give you a way to do that- activating a wand is (at least) a standard action.
the rod of splendor says that one must "hold or carry" the rod...so if I were GM in that instance I would allow you to just have it on your belt or strapped to you somewhere.
meta magic rods must be "in hand" when you wish to use them
when you perform actions in RPG's like pathfinder they are suppose to be very thematic...activating a wand is a standard action, so mechanically you can only activate one wand at a time (once per round unless you somehow get more standard actions). Thematically you are concentrating on the specific action/words used to activate that one wand
Also from a mechanics/balancing standpoint its ridiculously OP to allow someone to activate a bundle of wands at once.
The game falls apart if you allow multiple items to be used simultaneously, or to be used in a way that seems to negate the normal action economy for holding, retrieving and storing items.
Should you be able to glue 6 vials of alchemist fire together for 6x damage? What if you get a really big glass vial that holds ten doese at once? 10x damage?
Should you be able to glue 2, 3 or 6 wands together, then just hold the bundle and decide on a turn-by-turn basis which one to use?
No, you shouldn't be able to do it. And not because there is a specific rule saying you can't. PF doesn't tend to work like that. The rules tell you what you can do - like that you can use a standard action to activate a wand held in your hand. So that's what you can do. Period. The rules don't have to tell you that you can't hold 20 wands with the same command word, in one hand, and fire them at once.
If you as DM want to rule that you can superglue 2 wands together and selectively activate them whilst holding them in one hand, hey, no problem. I completely agree that the bottom line is the fun folks have playing the game.
I'm just saying that you won't find any rules specifically allowing or disallowing such antics.
And if you allow that, where do you draw the line? Why can't I creatively pour 100 vials of alchemist's fire into a small barrel and roll it down the hall for 100d6 of fun damage?
The rules don't expressly allow you to do it, and an interpretation of the rules that does allow it, while manageable, is sufficiently weak that it amounts to asking for GM permission anyway. As an aside, if wands could be glued to things, we wouldn't need a spell to stick them into swords temporarily.