Hi Michel,
Normally, when you have a phone with dual SIM, there is always one SIM that takes precedence (the most used of the two that you will have to choose when the phone asks you), so simultaneous. But you have to know that every time you are going to call or send a message, the phone will first ask you which SIM is going to be used and you will be able to choose (so both SIMs are "active" but never at the same time!)
Greetings