Can I connect two outputs to supply more voltage to speaker?
Yes, you can double the voltage even if two outputs share the same ground, if you do not use the ground, but rather take as your output the voltage difference between the outputs.  Of course, if the outputs output exactly the same thing at the same time, the voltage difference is approximately zero, which isn't useful. What you must do is invert the signal to one of the outputs, so it goes low when the other swings high and vice versa.

This is called bridging, and is commonly done with audio amplifiers.

Bridging doubles the peak-to-peak voltage seen by the load, such as a speaker. As a bonus, it usually eliminates the need for a DC-blocking coupling capacitor or output transformer.

The reason it doubles the peak-to-peak voltage is because it uses the difference between the outputs. A single output can only express a voltage from 0 to V. But the difference between two such outputs can range from -V (0 - V) to as high as +V (V - 0). From -V to +V, we have 2V!

This is just the general idea, minus the background work to determine whether, or how, this is safely applicable to your device.

If you take two outputs in parallel, which produce the same signal, you do not obtain an increase in peak-to-peak voltage. However, you get more current delivery capability, which you can exploit by using lower impedance load to develop greater power.

And of course, you can always drive two speakers with two outputs. Two speakers side by side with the same signal at the same volume deliver twice the power, which is 3dB louder.