No only the 1 currently active Watch will ring. While you can pair up to 3 watches to your iPhone, only 1 can be active at any given time. For your testing you will have to do it one at a time. They can't both be active at the same time.
Active Watch means the one that is on your wrist. You can manage the behavior that determines which watch is Active by selecting the 1 you want to be Active manually: open the Apple Watch app on iPhone, tap My Watch, tap your Apple Watch at the top of the screen, then turn off Auto Switch and pick your watch or you have have Automatic switch active(default) just raise your wrist to make it Active.
Dave