If you want to work in stereo, you'll want a stereo amp. The 12S makes it quick & simple, offering much greater simplicity than running pairs of everything, while also offering a few extra capabilities that go further too. You'll find that it helps get the logistics out of the way for a much clearer path of experimentation, and that working in stereo can make for a much richer sound.
The audio inputs (bottom) will detect when either jack is unused and automatically duplicate the signal to the other channel, allowing for use with mono sources. Similarly, when either envelope jack (just above) is used alone, the envelope signal is automatically applied to the unconnected channel as well. So, you can connect a stereo input and have both channels controlled by a single envelope generator connection.
On the other hand, you can also connect two different envelope generators for a more complex stereo image or to use the device as if two parallel amplifiers. Gain controls at each envelope input allow you to magnify, decrease, or invert the envelope behavior. Just above, a volume control serves mixing duties before the signal joins others at the main outputs. Being modulatable (the jack just above it) with a dedicated attenuverter, means that you can also readily modulate volume changes and create tremolo effects.
Since stereo modular setups often involve two of everything, such as two oscillators going to two filters and so on until th 12S, you might find that the stereo spread feels too wide and the channels feel too independent. In that case, the (upper) linkable bleed controls allow you to blend varying amounts of Left into Right and vice versa. With the "L" (link) button engaged, the bleed of one channel is proportionally applied to the other.
At the top, you'll find stereo outputs, though you needn't use both - you can just use this as a mono amp knowing that you can scale up to stereo without having to replace so many devices. And, be sure to check out its cousin, the 116A Amp for polyphonic signals headed to a stereo destination.
The 12S Stereo Amp is a great go-to for working with Left & Right channels.