If you connect the two in Series, you will Add the Two Impedances, Thus Reducing Power.
If you connect the two in Parallel, they Will Draw more Power. Maybe Too much for your Amp.
While Both these changes can result in a Volume Changes, They are Only MINOR amounts of Volume Difference.
And IDEALLY Both these speakers should be Identical. "Same Speaker and Impedance".
To Comment Further, I would need to know More about these speakers and your Present amplifier.