I haven't done any research into it, Moses, but it has been said quite a few times on Facebook over the past 24 hrs or so, and I believe it to be true, that this is the first time since the 1950's when they were still in military use.
After the 50's, besides "FIFI", the only other B-29 that flew with any sense of regularity was "Fertile Myrtle", in the 1970's, and it never flew with "FIFI" and it never flew much - it was only ever marginally airworthy and never flew past 1980 except for a rather scary attempted ferry flight after Kermit Weeks bought it. Until "Doc" first flew last year, there were only two other B-29's that I'm aware of that flew at all since the 1950's in addition to those already mentioned - one being the Duxford-based IWM B-29 "It's Hawg Wild" that was flown from the US across the Atlantic on a ferry flight in 1980, for which it only flew the legs of the ferry flight and never was intended to fly more than what was needed to get it to Duxford - and the other being the March AFB static B-29, which David Tallichet flew into March AFB on a one-leg ferry flight with the landing gear locked down.
Yesterday, in addition to the race track pattern the B-29's were flying, they did join-up in formation (as my previous post link shows) and had a B-25 camera-ship leading the formation, so you know there are going to be some great air-to-air photos published in all of the warbird/vintage aircraft magazines of the B-29's flying together.