No. Page 14 says a hero can only have only order token at a time.
But if the first hero had not used his Aim token, the Guard token would replace the Aim token.
Actually... the full thing reads
Descent rules, p.14 wrote:
A hero can only have one order at any one time. A hero that has already been issued an order cannot receive another order until the first order is used or removed(see below).
And then it lists each order and when they get removed. There's no mention of an order being replaced, or even removed by choice.
This would lead me to conclude that, in your example, the Guard token couldn't be played at all on a hero who already has Aim, period.