Murano/Burano - is a half day possible?

We did both, but we were in Venice for 5 days. Retrospectively if we were pressed for time we would have just gone to Burano - becasue it is so different. If we had even less time in Venice we likely would have just stayed in Venice.
As for how much time you spend in either one is up to you.

I also prefer Burano to Murano. It's a bit further away, but less visited.

A half day for one of those would probably work but a half day for both would be really difficult if you want to spend any time there. I agree with the others that we enjoyed Burano much more, Murano is mostly about shopping for glass. I think it takes about 15 minutes to get to Murano and 45 minutes to get to Burano.

With only two full days in Venice it would be difficult to spend a majority of the day at the other islands in the lagoon without compromising some of Venice. I have been on two different 4 night trips to Venice and that worked well for us to go out in the lagoon. I would look at some pictures of each area and decide what are your must do activities in Venice and see if you have any time remaining. You can also wait until you are in Venice to make the final decision since you will not have to prepurchase anything for the trip into the lagoon.

I would leave it for your last day and make your decision then about whether you want to do both. I looked at my journal from when we were there in 2008. It shows that we went to Murano in the morning, then went to Burano where we had lunch. (We saw a glass blowing demo on Murano and walked around the town) After lunch, we explored Burano. So it took us more than the morning, but we still had time to go to St. Giorgio Island and do the slow vaperetto all along the Grand Canal before dinner. We are not much for shopping so probably did the islands a little faster than someone who loves to explore all the shops. We only spent 3 nights. This was our second trip to Venice. In 2003, we spent 3 nights and only did Murano. As I said, I would do everything else you want to do in Venice, and then decide about the islands. It does not require any reservations or anything so no pre-planning necessary.

One more tip. On our first trip in 2003, we tried to catch the vaperetto for the islands right on the waterfront by San Marco square. It was so crowded and pushy and hard to get on. In 2008, we walked to Fondemente Nuova stop and had no problems getting on a vaperetto. So much easier and not stress or pushing. Use a map (a good Venice street map should be your first purchase when you arrive...you will use it constantly and get your moneys worth) to cut through Venice (it would be long walk along the waterfront, but not far if you go straight through Venice). In addition to catching the boat, you will have a fun time wandering down canals many visitors never see to get there.

I agree with what everyone else has suggested. I just want to add that a lot depends on what you are hoping to do in Venice. There are a lot of fine museums, and after two weeks in Italy, you might have reached museum saturation. And that's okay, because Venice is such a wonderful city to wander around in without even if you never go inside any buildings. (Well maybe you should visit the Basilica of San Marco at the very minimum...)

If you are okay with a more laid-back approach to visiting Venice, you can probably find time for both Murano and Burano. If not, I agree that Burano is the better option.

I would also say, however, that when you get home you will remember the things you saw, not regret the things you didn't see. So it doesn't really matter what you skip, as long as you see the things you love and love the things you see.

You will have at least 2 hours on the vaporetto if you include both, so unless you are speed walkers and don't want to look at anything as you pass, no I don't think that you can do both in half a day.

Then if you add an hour or so at the Glass Museum - well worth seeing IMHO - and then if you are at Burano, again well worth it, but not fast, it would be a shame to miss the fabulous ancient church with the fantastic mosaics on Torcello. Torcello is reached by a short shuttle vaporetto ride to and from Burano.

Good advice above. We typically find it takes us 6 hours to visit Murano (arriving at 10:00 when the shops open), then on to Burano and Torcello. We lunch on Burano. Typically we leave our digs about 9:00AM and get back about 15:00-16:00. Well worth it, IMO. Far better, on a sunny day, than going to another museum. The church on Torcello is beautiful and it is always nice to get away from the cruise ship groups in San Marco. If you have to skip something, skip Murano, though it kills me to say it. And Nigel is right: The glass museum is very good.

Travel time to Murano/Burano can vary a good deal depending on where you board the vap. From your hotel it might be quicker to walk across the main island and get the vap. rather than just taking it from the nearest stop. Hard to say without knowing your hotel location. It can probably be done in half a day if you start early. A full day will be much better.