Dual SIM Samsung S9 on AT&T

I am based in California, USA.  I inquired with AT&T tech support to understand if they can activate a dual-SIM Samsung S9 unlocked phone, that I am looking to purchase.  I am hoping to have one phone, two numbers (one for work, one for business).  I'm noting the Samsung Dual-SIM S9 model (BTW the model is different than most S9s which can use only one SIM!) has two physical SIMs in the tray, unlike the new iPhones (XR, XS, XS Max) which have one physical SIM, in addition to an eSIM (chip on the board, wave of the future I'm reading!).  AT&T says yes, they can activate it.  The impact on my plan is like adding another line, i.e. additional $35/mo. for the number and data.   

My second call to AT&T, my further inquiry if the mode for the dual SIM would be Dual-SIM-Dual-Standby (DSDS) or Dual-SIM-Dual-Active (DSDA), a concept which I'm only learning about.  I'm not that clear on what the precise DSDS/DSDA definitions are, but per AT&T tech support, if you're on one call and an SMS text comes in on the other SIM, it won't show up until you end that call (but implying it will show up!).  If you're on one call and a voice call comes in on the other SIM, it would be routed to VM.  That sounds like DSDS to me (perhaps someone can add to this?), and anyway each scenario sounds acceptable to me.  (I'm not planning to conf the two lines together anyway  

If anyone has any additional information to the contrary or otherwise, please post!   As for me I'm going to go ahead and buy the phone, I'll try to remember to update this post with my actual activation and use experience.