Wt writers' big mistake was to make the 1914 generation the "anointed". Had they stuck with the likeness of the first fulfilment, namely the general public, some of who survived the end [of jerusalem], watchtower could have coasted to 2032 , because there is always a certified 118 year old on the guiness book of records. 1914 + 118 = 2032.
wt anointed on the other hand, have to be ~20 year olds, like Fr. Franz was in 1914. Therefore, wt's "anointing" mistake lopped off 20 years of the generation, which is fine if you gun for a "ssooon" armageddon date. so:
it was the wt's anointing of the 1914 generation that required the overlap grouping. but
The real big, real mistake in the "anointing" is of course that in detail it is absolutely unscriptural.
The bible generation can only pass after all has occurred. The wt generation, the partakers, has to pass before everything serious occurs, to be in heaven to cause the havoc. aka GT.
no wonder they now work on a personal relationship with the deity, because their ever tacking, tacky doctrinal ship is shipwrecked