It means you'd have it programmed to two different phone numbers which is virtually impossible.

Samsung allow simultaneous service mostly. But may be it will not be allowing for your model. That's why you are not able to do that. You can contact costumer care for more details.

Um... honestly you shouldn't be able too at all O.o How do you even FIT 2 sim cards into that thing? It's only suppose to be able to take ONE... and the fact is the phone (and phone number) can only be registered to ONE sim card... I used to work for a cell phone company and I know this for a fact, if you activate a new sim card to a phone number (or data number) the old one becomes PERMANENTLY disabled, you can never activate it again and that's just the way it is. Each Sim card is programmed to a DIFFERENT phone number so to be able to use TWO to the same device is insane. It means you'd have it programmed to two different phone numbers which is virtually impossible.

Help!
I have a Galaxy S6102 and I can not use two sim simultaneously on it.