You ask why not. You seem to want both of them active.
1) Having both active would use a roster spot that Belichick thinks is needed elsewhere. After all, he has had one of them inactive for many weeks. One of our top special teamers was inactive last week. I certainly would rather have Jones active than have both Tyms and Dobson active.
2) Yes, my preference is to give the 3-5 reps to Wright instead of Tyms or Dobson. There is only one ball. If Tyms or Dobson is on the field, another player comes off. I would note that this is simply my preference. Josh and Bill disagree. They believe that the Tyms/Dobson option is important to use a few times a game. For me, this doesn't keep the defense honest unless the players actually succeed once in awhile. For me, having Tyms in the lineup instead of Amendola limits rather than increasing our play calling options. Part of problem may be the ineffectiveness of Tyms. I just don't see him as better in this minimal role )if there is to be one) than Thompkins or Dobson.