I'm not sure about that. Normally the EA receives OnTick (and other) events for the currency where it is added on - when a tick comes for the current currency, OnTick() is executed.
In theory, if you really want to control all currencies the same way under the same EA, you should have some infinite cycle to detect tick events by all currencies. And this is not impossible, but when you have to work with some currency, all the others will wait. This is called "blocking".
Actually I don't know what will be the behaviour of such an EA, I never tried this. This is something that I had an idea to make, but it's still only an idea. I guess that if you have multiple EAs they does not wait for each other.