"Is it doable to see the two castles in Fussen and Linderhof in one day?"
If you only want to look at the "castles" and add them to the list of things you have "seen" so you tell your friends, yes. If you really want to experience these palaces, then no! The tours of the castles in Hohenschwangau are limited to about 45 minutes each, but you have to pick up your ticket an hour in advance, and then there is maybe an hour or more walking from Hohenschwangau to Neuschwanstein and back, so figure almost four hours, if things go exactly right, in Hohenschwangau. Then there is an hour's driving time from Hohenschwangau to Linderhof. But in Linderhof, after you tour the small palace, itself, there are a lot of other things to see, such as the grotto and out-buildings. I spent 6 hours exploring the grounds and still felt I hadn't seen everything. I think to try to see all three in one day is pushing it.
If by "Mehrtagesticket" you mean a 14-days-Ticket from the Bavarian Palace, Lake, and Garden Administration, note that Hohenschwangau is not included. The online system for reserving "King's" tickets for the castle does not seem to have a way to indicate that you have a pass. If you make reservations online, and pay for the tickets, will they refund the difference if you show up with a pass? I don't know? But if you call the number for reservations, they might allow you to use the pass.
