I wouldn't reccomend that! They are your snakes, but there are several reasons why I wouldn't do this:
First off, it is like putting a puppy in with a hippo. The hippo wouldn't eat the puppy, but sheer size and clumbyness is a huge issue. There is a very big chance your milk would get injued. If not at first, the boa could hurt it at any time, whether it means to or not.
Second, milks are naturally snake-eaters. Just because the boa is too big doesn't mean that the milk would not be constantly upset though smelling food all of the time. In one of the previous nature centers I worked in, we rescued a california king. It was in a room with several other snakes, although it had its own tank. It would always bite in a feeding response. We adopted him to someone who had no other snakes, and the problem went away. The very smell of food makes the lunatic milk and king snakes really funny when it comes to behavior.
Third, the mixing of different species is not such a good idea. Atleast your boa and the python were similar in size and care (and I, too, might have considered very temporary housing together), but each snake carries its own bacteria and disease resistances. There are some things that a corn snake may be resistant to that could kill the boa, or vice versa. Also, are both of the snakes captive born and eating disease-free prey? Even so, they could be holding a parasite load that could transfer between the animals. What might be okay for one animal might harm another.
Fourth, the care requirements for a milk snake and a boa are very different. The temp is one concern, but have you thought of the humidity requirement? The milk snake, although I haven't reserched much the specific species, I think has a far lower humidity requirement. Exceeding this can cause blisters and other damage to the snake.
Fifth, I tried something similar once when I was younger and just learning the trade of snake keeping. I got a ball, and put her in with my spotted python. I thought they'd be fine. Both were well-fed, but the spotted got very defensive within a few minutes, and grabbed and bit the ball. It wasn't a very nice experience, and proves that even similary sized pythons can have issues. However, i now keep the same ball in with another ball, and they are fine. I would always keep it to the same species with snakes.
If you are looking for a way to conserve heat, why not calculate exactly how much your snakes are costing you? Check out this link:
http://www.anapsid.org/electricitycost.html That site will show you how to figure out exactly how much the bulbs and/or eating pads are actually costing you down to the cent.
