Hey,
so we all need a bellsprout or Bulbasaur to get into the Gym, why not use second trainer and let bellsprout fight with us, that saves a turn on onix and some times even one round on geodude, and if we catch it with the lure we get in the Forrest it’s every time strong enough so that we don’t need to “search” for good one.
What do you all think? Is it a time waste to get the 2nd trainer out?

A double fight turn takes roughly the same time as 2 single fight turns.

Geodude, you can probably finish in 1 turn, which is equal to 2 turns with Eevee.
Onix, you can't finish in one turn with Bellsprout. Therefore it takes the equivalent of 4 turns with Eevee. In single battles, Eevee only needs 3 turns to defeat Onix.

In Pikachu, this is different, since Oddish can defeat Geodude without Pikachu even doing anything most of the time and Onix also only needs one turn, sometimes with Pikachu not even needing to deal any damage with Double Kick.

Hmm, thought always it was just a tiny bit slower but sounds logical ^^ thought the time to get 2nd T und to send him away costs more time then the extra turn he needs