Some guitar amps do give you the ability to add an extra, external speaker to its main internal speaker.
Why would you like to try this?

~The hear how a differerent speaker responds to your amp.

~To spread the sound of your amp.

If your amp does not have an external speaker input you could try to connect your amp to another amp to achieve a similar kind of idea. When you are connecting your guitar to both amps set the amps up for a similar kind of tone. You will hear your sound as one amp rather than two different amps. Take out the sound of one amp to hear the differences in your sound, you will be surprised by what you hear.

 

Happy playing and hope to catch you soon again,
Eddie