NOTE: You probably should have posted this in the
beginner section.
Ch1 and Ch2 are isolated to give you the flexibility to use them independently, but you can also tie them together in various ways (series, parallel...).
In your case, tie your circuit ground to the (-) terminals of both Ch1 and Ch2. You can do that any way you want (e.g. tie Ch1 (-) and Ch2(-) at the DP832 then bring a single wire to your circuit), then use Ch1 (+) and Ch2 (+) as your 2 supplies.
Whether you also connect the DP832 chassis ground (which is earth main ground) to your circuit ground is an independent question. In general, there is no need. Regardless, it doesn't functionally change the behavior of the DP832.