While most people manage quite well with a single SIM card, if you want better demarcation between your work and home life, or you want to make roaming with multiple SIM cards easier, then having the extra SIM already in your smartphone is going to be a real timesaver.
See also: Turn your Apple Watch or iPod touch into a cellphone
But how do you install an extra SIM or two into the iPhone 6s? After all, that SIM tray is far too small to accept another SIM.
Fear not! Here's how you do it. And don't worry, as no jailbreaking is required.
Enter the SIMore X-Twin 6S Plus, a dual SIM device that allows you to have 2 SIM cards (1x nano SIM, and 1x standard SIM) in your smartphone. The nano SIM sits in the SIM card tray along with the tail of the X-Twin 6S Plus adapter so that it makes contact with the SIM card pin assembly inside the handset (yes, that ribbon cable is thin enough to fit between the SIM tray and the body of the iPhone 6. Then the other SIM card fit into the adapter.
Now you don't want to be walking around with a ribbon cable hanging off your handset, which is why you pop it all into a supplied case, which keeps everything tidy. The case also protects your handset and also servers as a holder for additional SIM cards and a SIM tray popout pin.
And that's it. You switch between SIM cards from the iOS interface by going Settings > Phone > SIM Applications.
The kit, which includes the adapter and a case for your iPhone 6s, is €35 (around $40). If you have an iPhone 6s Plus you can still use the kit but you'll need to find a flexible rubber case to house the adapter.
There are also versions available for other popular handsets, such as the Samsung Galaxy S5 and S6.