This Old Goat agrees with OldGoat (above) that it is simply a coincidence that both cults picked the same date.
The thing that matters is HOW they arrived at the date 1975, and the Watchtower's Bible-based© Chronology provided the magical arithmetic framework for the selection of 1975 since at least 1918, when H. W. A. was about 20 years old, and it wasn't until the late 1930s that the WWCOG was established.
HWA's theology was largely influenced by the Church of God, Seventh Day which is a distant cousin of Watchtower theology.
In 1931 Armstrong became an ordained minister of the Oregon Conference of the Church of God (Seventh Day).
1975 in Prophecy was a warning to the reader of what was scheduled to happen after February 1972 . The timeline was uncertain and, although the title of the booklet was specific, 1975 was not mentioned in the text in relation to Biblical prophecy. All specific dates within the booklet were in relation to events or outcomes not specified by the Bible. The biblical prophecies are ambiguous as to their timing.
The booklet was written in 1956 during the Cold War years. It stated that the United States of America, the United Kingdom, Australia and other English speaking ("Israelite") nations, contrary to popular cultural belief, would neither be attacked nor destroyed by the Soviet Union, but that a nuclear World War III would destroy these countries. The attack would come from a German dominated United States of Europe led by a Nazi-style dictator (identified as The Beast), and dominated by a religious leader who would probably be a Roman Catholic Pope identified as the Antichrist.
In the aftermath of the nuclear attack one third of the populations would be dead. Another third would then die as a result of simultaneous attacks from abnormal weather patterns which would create drought, destruction and epidemic diseases. The remaining third would then be taken into slave labor camp captivity by the United States of Europe.
http://www.cgca.net/pabco/5proph.jpg
[above, an illustration from 1975 in Prophecy. The illo is the work of Van Wolverton, who followed HWA and illustrated horror comic books for a living]
Thanks to wikipedia for most of the content above.