
Dryades |

If I Boost Eidolon for my Act Together action, can I squeeze in that Extend Boost?
I assume no because I think, as per simultaneous actions rules, you can only squeeze in a free action in the middle of another action if it has a trigger, which Extend Boost doesn't have.
Additionally, to rule out as much as possible, I couldn't just Extend Boost outside of the Act Together either, huh? Because Extend Boost wouldn't recognize the Boost Eidolon inside Act Together as the next action?

Darksol the Painbringer |

By RAW, you're right, since Act Together is an activity separate from Boost Eidolon, and Extend Boost requires that your next action is Boost Eidolon, of which Act Together isn't.
That being said, I'm not so certain that is the intent of Act Together, given that it is largely so you can utilize both characters on the battlefield simultaneously, and isn't meant to diminish combination effects between both characters; otherwise, the Tandem trait is more restrictive than necessary.
There is also the issue of you doing actions within actions in this case, since Act Together costs 1-3 actions, but then lets you do 1-3 actions worth of actions, with the other taking only one action; an argument could be made that, because you are doing Act Together to perform 1-3 actions, if your next action from those 1-3 actions is Boost Eidolon, you're still meeting requirements for Extend Boost. After all, just because the activity says you perform 1-3 actions doesn't mean that you can't take Free Actions before or after them. (Otherwise, then you couldn't Release an Object mid-Act Together, which just seems absurd, and that is just one of many options.)
I also don't see a balance issue here, since the worst thing that can happen is that the character burns a Focus Point to Extend Boost and either fails (in which case no effect this turn), or applies the effect and the Focus Point is expended.

breithauptclan |

There are some other threads about this too (example thread). Act Together is a bit vague on exactly how it interacts with subordinate actions and the differences between an action (one simple action that you can take), an action (one of the three actions that you get at the beginning of your turn), and an activity (a complex action that may include other simple actions, or may only be one action but is defined as an activity - such as spellcasting).

Dryades |

There is also the issue of you doing actions within actions in this case, since Act Together costs 1-3 actions, but then lets you do 1-3 actions worth of actions, with the other taking only one action; an argument could be made that, because you are doing Act Together to perform 1-3 actions, if your next action from those 1-3 actions is Boost Eidolon, you're still meeting requirements for Extend Boost. After all, just because the activity says you perform 1-3 actions doesn't mean that you can't take Free Actions before or after them. (Otherwise, then you couldn't Release an Object mid-Act Together, which just seems absurd, and that is just one of many options.)
Hmmm, so you suspect Act Together works more like how the Command an Animal or Sustain a Spell actions make a minion? As if the act of Act Together simply functions as the permission for the Summoner and Eidolon to take a set of regulated actions, just like how commanding or sustaining gives the minion permission to take its 2 actions? I never thought of it that way, but it makes it a lot simpler to visualize for me actually.
Also now I'm thinking, is Act Together technically an action/activity that the individually Summoner takes or is it, by virtue of being a Tandem action, taken by both the Summoner and Eidolon at the same time? Like, did we both "Act Together" or did I, as the Summoner, "Act Together" but the Eidolon simply did whatever action/activity they took. It's probably irrelevant as I don't think Eidolon's can get metamagic atm, but I'd probably ask the same original post question for them if they can in the future.