I couldn't find it in the book, but it just occurred to me that my players are spending Advantage on multiple results.

Say they got 4 Adv, they could activate a Critical Hit for 2, Recover 1 strain, AND pass a Boost onto the next player.

Is this allowed?

Yep, you can spend those 4 Advantage as listed in your example.

Yep. All good.

I couldn't find it in the book, but it just occurred to me that my players are spending Advantage on multiple results.

Say they got 4 Adv, they could activate a Critical Hit for 2, Recover 1 strain, AND pass a Boost onto the next player.

Is this allowed?

Yes, and honestly as you advance with skills, talents, and better gear, it's pretty much a non-negotiable, because you have so many Advantages lying around on some rolls.

it's also worth mentioning that you can spend Advantages to trigger a critical injury or recover strain multiple times. But things like passing boost dice, disarming, or increasing our melee or ranged defense can only be triggered once per check.

So if a very-skilled PC using a heavy blaster pistol generated 8 Advantages on their attack roll, they could spend 6 to trigger a critical hit (presuming they hit in the first place) twice, getting a +10 bonus to the percentile roll as well as recover 2 personal strain.

Now they couldn't spend those Advantage to grant themselves more than 1 boost die to them specifically for their next check. Although they could spend 3 Advantage to pass a boost die to themselves and the next PC to act, and would get two boost dice if they were the next PC to act (can happen due to being the last to act in the current round, and then the first to act in the following round).