One effect....

[b]

[/b] etc...

Seems pretty clear, but the thought occurs of the following situation...

(Say)
1)You have a constant effect on (say) a sword {Edge of the Razor}
2)You have a touch effect on the same item {when item touches X}-Duration effect
3)An effect that the user can trigger (say) {Bolt of lightning} to be used when a strike is successful...

Seems the above rule means that only one would take effect...

How does this work out...?

I've always read the word "use" as meaning "choose to activate". Obviously, if an item has 1 effect that protects from wooden weapons, and another effect that protects from metal ones, you don't have to choose as the bandits pound on you.

If effects are designed to go off automatically, whether linked to each other or outside magical triggers (see rules), or are already running, no prob.