The Techter Skill Extend Assist reads that it increases the duration of Shifta and Deband to 180 seconds and does not state that it requires Techter being the main class. When cast on my Force/Techter, the duration of Shifta and Deband is only 60 seconds even after standing in the Technique for the full four ticks.

Extend Assist does not require main class, but Long-Time Assist does. If TE is not your main class, you get only 15 seconds per tick on S/D, not 45 seconds.

If you want 180 seconds as a sub TE, then you have to cast each buff 3 times.