And may it be the first of many. Welcome to Flyertalk flyingAllOver and welcome to the BA forum. It's really good of you to join us here, welcome on board and please make yourself at home.

That isn't a DUB specific MCT. BA.com has programmed into it that "MCT", since 2 hours covers most places most of the time. I guess they did that rather than building a whole lot of changeable data which is sensitive to the trips and airports involved. So no, you can't do it online, so I guess you will need to call up and see if they will do it over the telephone, I don't think they should charge a fee here, but I doubt there is much consistency on that point. Otherwise book 2 singles, if you accept the risk you are taking.

The actual MCT at DUB is 45 minutes, though BA enforce 2 hours for various codeshares such as the US/AA flights.

Since the airside connection route is generally slower than the landside process, if you are at the front of the queue leaving the aircraft, you should more or less know how to do it: out to landside and then up to departures, crossing well over to the right to get fast track.