You'll need to feed them in the brooder stage obviously, then in winter they should have a little supplement while the grass isn't very nutritious.
You can leave them out 24/7, the only problem being predators. I trained mine to "go to bed" in a shed which I locked at night. But, now with 30+ on the ground the flock is large enough to not worry. But, with two, they won't have adequate protection against predation.
As far as weeding goes, you need to be very selective in how you use them. I let mine in the potato patches. But, they don't go in the main garden. They'll eat any lettuces, spinaches or other leafy greens you have while ignoring the weeds. So, plant your leafy greens in a separate area if you want to use them in the garden.
It's not a huge concern, but I've also watched my geese gang-up with 3 or 4 of them pulling corn stalks down to eat the leaves. It's funny as all get-out and I generally tolerate this for their enjoyment.