My opponent has two Ondu war cleric. He taps them both and said he gains 4 life. But I believe he couldn't use the second one with only two of those cards, because cohort said you need another untapped ally
He can only gain 2 life
When you activate an ability, the first thing you do, is pay it's cost.
602.1a The activation cost is everything before the colon (:). An ability’s activation cost must be paid by the player who is activating it.
602.2. To activate an ability is to put it onto the stack and pay its costs...
In this case that means that you need to tap the Ondu war cleric and an other untapped ally. They will be tapped before the ability is put on the stack so you won't be able to then activate the second war cleric as he is already tapped.