It seems you are implying that your receiver itself has got an amp though small. You use another amp to boost your power, is that right?
It will definitely damage both your receiver and amp, just waiting which one to go first if you do what you are saying. In the first place how do you connect your receiver to the second amp? it has a line out i suppose? and for now your speakers are only connected to the booster amp? So it is only a matter of selecting where your speakers get connected when you wanted. DO NOT, in any case splice your speaker wire to attach your receiver speaker to the output of your booster where your speakers are connected.
What you can do is just make a switch selecting which source you will use for your speakers. You just need a simple toggle switch 2poles/double throw. 1pole for each channel (R/L) connect your center pole to speaker, the other selectable 2poles to the output of booster, the other pole to the receiver. (each channel) that is your safest. If you a a techie you can actually make a power driven relay switch to select the speakers where to get connected but maybe you can not do that on your own. it has to be someone with knowhow, simple design though. the same principle, your meccanilca 2pole/double throw switch will be replaced by the same specs but relay. elecically driven whrn you turn on/off your booster.