I am pretty sure any review of a two day trip isn't going to be very good. There simply isn't sufficient tie to do proper visits in such short time frame. By the time everyone on the excursion is picked up at their hotel its well after 9 when the bus departs for Delphi. So that means you are not getting there until after noon because there is a rest stop on the way. That gives you a few hours at the site. Then you have another 3 hour drive to get to Meteora for the night. You will get some time there the next day [probably from 9am till 3pm before the buses depart for the 5 hour drive back to Athens. Two days is really too short a time period to do those two sites.
Perhaps you should look for a 3 day trip which will give you enough time.
Even if you tried to do it yourself by public bus you really won't have much time if you try it in two days. First bus out of Athens to Delphi is at 7:30 and last bus back is at 18:50 so that gives you quite a good amount of time for both the the museum and the site.
Next morning take the train or bus to Kalambaka. From what I can see they will get you there by 1 to 1:30. Then you have to take a taxi to Meteora. The Monasterys that are open that day close around 3 as I recall. Train back to Athens departs at 17:22. Not much time. To do Meteora properly you need a one night stay so three days.