POTUS election results thread

Can someone let me know how both Georgia senators are up for election/re-election in the same election cycle?

I don’t know of any states in which that occurs besides Georgia.  So, I am assuming that someone died in office or some other incident that caused a tenure to be cut short. 

And then, how is it decided which Republicans go up against which Democrats? Not to mention where all the other candidates fall out. Since there are no districts, like there are for Representatives, how is it divided up when they are all running during the same cycle?