Baking cauliflower with potatoes
With an hour in the oven at roasting temperatures (400-450 F/205-230 C), small pieces of cauliflower will likely overcook, not under. Most recipes call for only 25-30 minutes in the oven at this temperature. Under foil, the cauliflower will add a lot of liquid - steam - and may even get sort of mushy before you remove the foil. You definitely don't need to pre-cook the cauliflower.

You haven't noted your baking temperature or the size of your cubes... for that long amount of time, I'm guessing you're baking your potatoes at a lower temperature, as recipes for 1/2 inch (1 cm) cubes of potato don't generally require that long of a baking time... but most recipes don't call for cooking under foil for that matter.

Have you thought about finding a recipe (if only for the methodology, not flavors)? Seems like you should be able to ditch the foil, cook at 400 F/205 C for about 30-40 minutes and be done with it.