Looking at my router with two antennas, I have to ask; are they in phase or is there a particular shift for some directionality?
I don’t know that there’s a clean answer to this unless you give info on what router you’re looking at. A router with two antennae could possibly be doing some phasing stuff, but it’s probably likelier that one antenna is for 2.4GHz band and the other is for 5GHz.
If it’s 802.11n or newer( e.g. ac) standard of WiFi – it’s definetly part of the WiFi MIMO implementation. You need at least 2 of them, but majority of the expensive models have 3 or even 4.