Casseroles are typically frozen after they’re baked, so they just need to be warmed up, which is part of why they’re so convenient. Or they can be frozen after assembly and thoroughly baked at a later point. There are some caveats, of course, but for the most part either way is fine. Just be sure to pre-cook any meat, noodles, or veggies before putting the casserole together the same as you would if you were baking it right away.
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There is one important step to take ahead of time with the extra casserole regardless of whether you’re baking it in advance or not — and that’s to line the baking dish with foil before assembling the casserole. Use extra foil so that you can fold it over the top. Once the casserole is frozen you’ll be able to pull it out by the foil and store it in the freezer without the baking dish. Just be sure to wrap it in a second layer of foil to protect it from the air before putting it back in the freezer.
If you are baking the casserole ahead of time then you’ll want to ensure that it is completely cooled down before putting it in the freezer. This will prevent condensation from damaging the casserole and ensure that already frozen foods don’t thaw. You’ll also want to leave off any garnishes or toppings that could get soggy during the defrosting process. This includes extra cheese, breadcrumbs, crispy fried onions, and cracker toppings.