This is something that's not high in demand for Apple users. I do believe there's a phone manufacture that has dual sim. If I'm not mistaken "BLU" smartphone manufacturer has this ability.
There are lots of manufacturers out there with dual SIM card capabilities, most of them Asian companies.
I am on vacation abroad and have purchased a prepaid SIM card because it’s cheaper than roaming. Whenever I need to call someone from my ‘own’ number I use WhatsApp or FaceTime Audio. This works like a charm.
My voicemail tells people that are calling me by normal voice call that they can reach me through WhatsApp calls or using the email that’s associated with FaceTime.
I have yet to miss a voice call or other important stuff.
I can see the benefits of a dual SIM card enabled iPhone but apparently it’s not something Apple seems necessary to integrate yet in their devices.
A reason for this may be that there simply is no room for the necessary chips for adding dual sim capabilities to the phone.