The two radio altimeters are displaying the same value; the co-pilot's unit is unsynchronized, and the captain's unit is not synchronized with the altitude corrected for QNH. I noticed that pulling the co-pilot's radio altimeter fuse (NAV No. 2) causes the red flag to appear on both radio altimeters. Conversely, the captain's radio altimeter fuse (NAV No. 1) does not work; furthermore, if both fuses are pulled, the flag appears, yet the radio altimeter continues to operate.