Sure thing!
Yes, you combine multiple of the 8 available voices into a single voice. The side effect is that you also reduce the overall number of voices available for other sounds while using one or more unison sounds.
For example, if you combine 4 voices to create a thick, lush pad, then you’re limited to only 4 other voices while that pad is playing (if leaving things in dynamic mode as opposed to locking the voices for the pad to ensure there’s always voices available even if it takes voices away from other sounds).
Creating sounds with unison generally makes them more lush, because you detune each voice and can spread it out in stereo. It also makes sounds louder on most synths. A tight but thick bass is probably fine with 1 or 2 voices. A dance music supersaw can often use 3 or 4 (or more). A dreamy, lush pad can use as many as 4 to 8.
If you run out of voices and trigger a new note, the voices will be “stolen” from whatever was previously playing, which can even stop that sound entirely. For example, if you have an 8 voice pad and trigger a new note, the previous sound will stop entirely as all voices are re-dedicated to the new note. In general, with only 8 voices, you want to be very careful and thoughtful about using even one sound with unison. Unless I’m using the Digitone for only one sound, I rarely use more than 2 voices in unison, and not more than 4, so that I can trigger the next note and not cut off the note that had been playing. But that’s me.