I haven't checked with the telco (Maxis) but is it possible if one (1) SIM card is shared with 2 phones? Meaning 2 phones with the same number can be used to access Whatsapp and make calls simultaneously. If not, can there be 2 SIM cards with the same number, 1 being duplicate?

Thanks for the response. I tried using Whatsapp on both phones last month and although it briefly worked, it's troublesome as I need to log off from Whatsapp on the main phone (with the SIM card) before logging into Whatsapp on the other phone (without the SIM card).

Also, the phone without SIM card will need to be connected to Wifi or hotspot, so it will be useless if there is no wifi or hotspot.

Unfortunately there is no flexibility to have 2 SIM cards with the same number to be used on 2 separate phones. Otherwise it would resolve everything.