In my garage can a refrigerator and a separate deep freezer be plugged into the same outlet
You'll want to find out how many amps that circuit provides and, if possible, how many amps or watts each unit can pull. In terms of watts, a 15 amp circuit gives 1800 watts, and 20 amps is 2400 watts. If your fridge and freezer aren't crazy old, there should be a label inside the door that tells you this. My freezer says it's 5 amps and my younger fridge says 3 amps (for comparison). Older devices may vary, depending on the size.

To find out how many amps that plug can give, plug something like a lamp in and flip breakers until it goes out. That breaker will tell you how many amps it is.