1. You can only put one other token on a doubler token.
RoP Page 13: Double a token end of paragraph 1:... You cannot
stack more then one token on another.
2. you can only use an agility token on the top token, a
doubler token is protected if covered.
RoP page 13: Double a token start of paragraph 3: You cannot
recast or flip a doubler token that has another token on
top of it.
edit: typo and add rules quotes
to add to this, if you flip a token that is on top of a double token, then you still double whatever the token is after it is flipped. so if someone has a damage/surge token on a double, if it is on the damage and you flip it, it is now a double surge.