Lots of great questions!
I don't believe that the Galway International Oyster Festival has yet announced their 2018 dates. I've had a little look and haven't been able to locate any just yet. The dates for this year are 22nd - 24th September, so that may help give some sort of indication. It may be helpful to reach out to them directly for better insight as to when they expect 2018 dates to be announced. The same also applies to the Dingle Food Fest.
I don't believe that they ever overlap in regards to dates, so I would be inclined to say yes that it would be feasible to attend both. I haven't attended either of these festivals myself, but have read some great things about both. I am sure that some of our other members will have had the opportunity to visit either of these festivals and will also be able to offer up some suggestions for you.
I am sure that once 2018 dates have been confirmed, it will make planning around the festivals much easier. Have you had a think about where else throughout the island you are keen on visiting during your trip?