The coldest day and night in any winter season varies from winter to winter. For Fargo-Moorhead, the average lowest low and lowest high temperature over the entire period of record 1881-2020 is -29 degrees and -13 degrees. For Grand Forks, where the record goes back to 1893, the averages are -31 and -13. For the most recent three decades, 1991-2020, the Grand Forks coldest low and high has risen slightly to -29 degrees and -12, but the Fargo coldest low and high has risen more, to -25 degrees and -10.
No, Fargo's climate has not warmed more than Grand Forks. The difference is due to the location of the temperature sensor at the airport on the north side of a city that has grown greatly to the south in recent years. A south wind has to blow over miles of furnace exhaust to get to the sensor, so any cold night with a southerly wind will read warmer at Hector Airport than at other locations, enough to inflate the averages.