If you buy the phone on payments it will be locked to the carrier. You will only be able to use SIMs from different carriers after you unlock the device. You will however be able to use two sim's from the same carrier.
What I want to know is if I can use the second SIM with my hotspot line as a data only connection so I don't have to worry about the 10G limit when I travel and need hotspot. I take my hotspot with me today but it would be nice to have a single device. I will test once I get a phone if I can convert my micro sim to a nano sim or have a new one made.