Munich-Fussen-Zugspitze connections

I guess that depend on how thoroughly you want to explore either site. If you want to take the tour of both castles (Neuschwanstein and Hohenschwangau) at Füssen and then take the train/cable car to the top of Zugspitze, I'd say not likely.

I think if you had reservations for the first tour of Neuschwanstein and then went directly to GaP, you might be able to fit in a short trip to the top of Zugspitze using the cable car.

For train connections, go to the Bahn website. Enter "Munich Hbf" and Fuessen Bahnhof" (Note the ue in Fuessen, not just u). Make sure the time and date are in European format.

There are frequent buses (RVO 9606) from Hohenschwangau to Garmisch-Partenkirchen. The schedule is here, or find it on the Bahn website. The bus goes directly to GaP from the bus stop in Hohenschwangau, the village at the base of the castles. No need to go back to Füssen.

BTW, according to the site for online ordering of Zugspitze tickets, a round trip for an adult is €45 (~$70). I think I would want to stay up there for more than a couple of hours for that price.

It takes a little over an hour and a half from Garmisch-Partenkirchen to the summit of Zugspitze using the Zahnradbahn (rack railroad) and Gletscherbahn. So, if you take the train all the way up, that's 3 hr RT. Add only 2 hrs at the top, and you have 5 hrs wrapped up in the trip.

It's faster to the summit (about 45 min) if you take the Zahnradbahn only to the Eibsee station and then take the Seilbahn (cable car) 10 min to the summit.

Under optimal conditions, if you timed everything just right and the moon and planets were in complete alignment... you still probably couldn't do it. From Munich in the off-season, visiting the castles takes the better part of a day, and it takes all day during the high season. Even staying in Garmisch-Partenkirchen, any meaningful trip to the summit of the Zugspitze takes at least 4-5 hours. So, if you want to see both, I would plan on two separate excusions.