I'd be careful with both your questions. My 5-1/2 week olds just spent one night in the grow-out coop when it got in the mid-20's Fahrenheit overnight with no heat. This was after a few nights out there where it got down to the lower 40's. So it is possible. But my grow-out coop is well shielded from wind and breezes and has extremely good ventilation. My ventilation is well above where they spend the night. They also grew up from Day 1 in a large brooder in the coop that had one area heated but the rest cooled down to ambient temperature, so they were acclimated. I also had 14 out there, so they could keep each other warm of they wanted to.
If your coop is well ventilated so the moist air can escape and they are not being hit directly with any breezes, they should be getting real close to being OK, even with only two of them, especially if they have some bedding to snuggle down in. I think turning the heat off during the day for about a week is a great idea to get them acclimated. If your coop is closed up tight where the moisture cannot escape, they are more at risk. If a breeze can hit them, they are even more at risk. We don't all have the same circumstances, but if the conditions are right, 5 week olds can do fine without heat. But if they are not acclimated or the coop is not right, they are in danger. I'd also be more concerned with bitter cold temperatures too. I'll admit to being a bit concerned with those mid-20's they just saw, but they did fine.
The two week olds are not ready to go without heat. If you put them out there, you need to keep the heat on for another three weeks or so.
Many people are successful integrating 5 week olds with 2 week olds. Your success depends on the personalities of the older chicks. Some chicks are aggressive brutes and will attack and kill smaller weaker chicks, but this can happen when they are grown as well. If you do decide to put the younger chicks in the coop (which I think is a great idea) I suggest you partition off an area for them that you can heat and feed and water them for about a week, where the older chicks cannot get to them. Some sort of brooder in the coop. After about a week when they have gotten to know each other, try removing the barrier when you can observe a lot, just to see how it goes. A little pecking and such is not a big deal, that's just the pecking order getting sorted. But if one goes after the young ones with malice, you need to rethink and regroup.
