If I have a figure that has two stamina, can I spend both stamina (reducing the model’s stamina to zero)?
Yes, there’s nothing stopping a model spending all of its Stamina (and it will usually regain some of that Stamina when it next activates plus via spells, equipment effects etc.).
Just remember that you cannot perform the same boost twice on the same Action but you can perform multiple different boosts to the same Action (e.g. you can’t perform 2 Power Attack boosts when making a melee attack but you can perform both a Power Attack and a Precision Strike boost).
Thanks. I thought it was that, but wanted to make sure. I’ll probably make a house rule that if stamina drops below zero then it is unable to perform actions until it is at zero or more (i.e. if it gets hit with frost damage and the stamina drain takes them below zero)