Stacy,
I don't know that you can't do it, but I think you would be extremely stressed if you tried. One thing we learned from planning 3 activities on 3 days, was that they take a lot more time than you think. Usually you have to arrive a half hour early and the activity takes longer than they say, such as getting everyone set up beforehand, and the time to take off the diving gear and put it away at the end.
The drives took longer than we expected, also. We didn't do the Glacier Ice Cave tour, but we went to a lava tube in the general vicinity. The roads were gravel by the time we got to the area which made for slower going. We try to be early for things, but we were late for that after driving from Borganes.
Could you do them on separate days? For example, do the diving and then see the rest of Thingvellir and the Goldn Circle that day? Gulfoss and Geysir don't take nearly as much time as Thingvellir. Then do the Glacier tour the next day and see sights around there.
By the way, we stayed at Hotel A near Husafell and would recommend it. (A with an accent means river, and it is on the White River.)
You could also contact the folks at the Glacier tour and ask them your question. They probably get questions like that frequently, since they are in a remote-ish area.
We booked with a self drive tour company, and they recommended against book in 2 activities in one day. And they both look like awesome activities. Give yourself time to enjoy each one, rather than stressing. I know different people who have done each and thoroughly enjoyed them.
