If my Ferrous Forerunner dies and creates two Mech tokens, can I give +2 to units from the played Mechs with the Rengar legend?

To answer your question: No, you cannot use your Rengar (Pridestalker) legend to give +2 Might to units.

Here is the breakdown of why this interaction does not work that way:

1. Rengar's Ability

According to your Rengar legend's text, its ability is: "When you play a unit, give a unit +1 [S] this turn." This ability provides a +1 Might bonus, not a +2 bonus.

2. Deathknell and Playing Tokens

When Ferrous Forerunner dies, its [Deathknell] ability triggers and is placed on the chain. When that ability resolves, it instructs you to "play" two Mech unit tokens.

Because you are "playing" units, this satisfies the trigger condition for your Rengar legend ("When you play a unit").

3. Resolution and Sequencing

Since you are playing two tokens, Rengar's ability will trigger twice (once for each token played).

• Trigger 1: You play the first Mech token. Rengar triggers, allowing you to give a unit +1 Might.
• Trigger 2: You play the second Mech token. Rengar triggers, allowing you to give a unit +1 Might.

Summary

While you can trigger Rengar's ability twice (once for each Mech token), each individual trigger only provides +1 Might. You can choose to apply both +1 Might bonuses to the same unit (resulting in +2 Might total to that unit), but you cannot grant a single +2 bonus from one trigger.

This is my interpretation based on the triggered ability rules and the card text retrieved. No official FAQ exists for this specific interaction.