As far as I know, no. There are however two ways to circumvent this problem that I can think of.
1) Add an intermediary node that will listen for the signals. For example, if you don't know who emits signal A, just use the _ready function of signal A emitters to connect to the intermediary listener, and then the listener will relay signal B to signal B listeners upon receiving signal A.
2) Find a way to make a node just listen for a signal directly, so that no matter who emits signal A, all signal A listeners will react.