Two amplifiers in single speaker with different frequency power
No. Connecting the output of two amplifiers to the same speaker in parallel is not a good idea.

Slight variances in amplified signal at specified periods of time produce a voltage (difference of potential). In this situation you'll have something similar to shortcircuit. 

Of course, you could add some series resistors, but this would modify speaker impedance and reduce output power, which isn't too much when using LM386.

You could wire them as a bridge configuration. Something like this:


Source: seekic.com

As you can see, it has only one input. So, you should use a simple preamplifier to mix the different frequency input signals.