Based on WAR this year: (before games on 9/22)
Seager: 1.8 fWAR, 1.8 bWAR
Anderson 2.5 fWAR, 2.4 bWAR
If we look at what they did in the past two years:
Seager: 2018 WAR (0.5 fWAR, 0.3 bWAR) 2019 WAR (3.3 fWAR, 3.3 bWAR)
Anderson: 2018 WAR (2.0 fWAR, 3.1 bWAR) 2019 WAR (3.5 fWAR, 3.7 bWAR)
Now, Seager was injured in 2018, which takes a big bite out of possible WAR that year. However, Anderson only turned in about 2 WAR that season, so it wasn't a big year for him either. Both players were pretty good last season, and both are good again this season, with Anderson having a slight edge due to having a better offensive season. Anderson leads in wRC+ 168 to 155, and in terms of baserunning and defense, both players are about equals this year. Based on success in the past year, and being an overall better player this year, Anderson has gotta be the one to get the diamond nod first. He has been as good or better than every shortstop in baseball this year, other than Tatis. Seager isn't too far off the mark though, and it would not be a crime to see him upgraded as well, however, there is no good excuse for a situation in which ONLY Seager is upgraded to diamond, and not Anderson as well.