Thanks for the quick answer guys !
I have another question involving Joust.
If i use one cube on The Templar to turn a white die into two red dice, may i keep (for this turn) one red die at my district and use the other one to activate Joust ?
No. The best way to remember is in this order:
Choose all the dice you will be using this turn.
Modify the dice.
Use the dice as modified.
So if you are going to turn one die into three with a modification, then you must choose (and pay any costs) for that single die, no matter how many it ends up being. If you need two dice which will be modified, then you pay the costs associated with using two, no matter the modifications, and so on.
Just like you can't buy dice from other players, and then decide to leave some behind, you also can't leave part of the modified dice behind.