Okay, the ability reads "Performs an Intense Focus maneuver, suffer 1 strain and upgrade the ability of the next skill check once"

sounds simple enough, but when it says upgrade it once, is that per intense focus, or can it only be applied once per action?

example: my turn: Action, shoot at an enemy, followed by using intense focus for my next ability. Next time its my turn I start by Intense Focusing, to upgrade my next ability once again, then take my action to shoot, with two intense focuses applied.

or could intense focus, spend 2 strain for 2nd maneuver, spend it on intense focus to upgrade next check again once.

tl;dr

Does "once" mean per action, or per intense focus maneuver.

The talent description reads, "Before making a skill check, the character may perform the Intense Focus maneuver. The character suffers one strain to upgrade the ability of the skill check once".

Based on that description I'd say that the Intense Focus manoeuvre is something you have to do on the same turn, immediately before making the skill check. Granted the description doesn't specifically state this, but I personally believe that's the intent behind the talent.

Of course you can always allow two manoeuvres, possibly also across two rounds. But in that case I would at the very least apply the same condition as with the aim manoeuvre; that if you take damage between aiming and making the attack roll you lose the benefits of having aimed.

I think it should be just once, unlike aim, which specifically allows you to aim twice.

I've been handling it as one single upgrade, and that you can't take a second Intense Focus maneuver to upgrade a check twice.

As Domingo pointed out, Aim specifically calls out that you can use it twice, so I'm thinking that talents like Intense Focus, Intimidating, or Side Step can only be used once on a character's turn during the round.

Hi

Aim (in eote core) says you can use two consecutive maneuvers to get two blues to the next combat check.

There are multiple other talents (like precise aim) that specify they can only be used once per round and a some that specify that the effect only lasts till the end of your turn.

Based on that, I'd say RAW intense focus can be stacked as many times as you like as it says neither once per round or only last until the end of your turn.

I'd ask your GM. The write up is clearly referring to the skill check which is an Action being upgraded and only once. If your GM wants this to simply be a new Maneuver akin to Aim but meant for other tasks not involving aiming hardly game breaking imo.