You asked about a fresh-air habitat...can't tell you. I've never used one...but the size you mentioned is big enough for one chameleon. I would never keep veileds any other way than one to a cage (except for those under 4 months of age or for mating purposes). One will usually decline in health and die earlier than it should when you do that. There is always low level stress there that will manifest itself as illnesses/health issues...and although the chameleon may not appear to have died because its sharing its cage, IMHO that is the real cause. Besides...do you want to risk having your chameleon for a short time by trying it?
I start off with my babies in screen-lidded tanks (yup...your read that right) that are about 20 gallons...but be aware that I live in a cool climate, and wouldn't recommend this under other circumstances...several to a tank. I do not use basking lights on these tanks. I separate any out that grow faster or slower than their siblings, or any that "argue" with their siblings and the rest stay together until they are about 4 months....depending on their growth rate. Because these chameleons mature sexually at a young age, its not "safe" to keep them together after the age of about 4 months. Also, I never mate a pair until the female is over 1 year of age. I like to make sure she is full grown before I put demands on her to produce fertile eggs. Some will produce infertile eggs before they are that old...but they seem to be less of a drain on the system.
You asked..."does it really need to be devided in half of will the two veils have enough room to find their own spots?" and "will using a mesh screen be acceptable, or is it better to make sure they do not see each other"...IMHO it definitely needs to be divided with something that they can't see each other through. Even if they find their own spots in the cage, there will be low level stress as they try to keep their own territory. In the wild they have more space to move away from each other, so that stress isn't constant. The male will also harass the female whenever she doesn't show her non-receptive/gravid coloration.
You asked..."ANy suggestions as to what bulb i should us?"...I usually use ReptiSun 5.0's...but there are other full spectrum (including UV) florescents that are much the same. You asked...4"Is 18 inch big enough?"...on the size of cage that you mentioned, it would be better if you had it longer (2')...and the fixture should be placed so that its about 12" above where the chameleon will likely sit. The further the chameleon is away from the UVB the less UVB there will be for it to absorb. There should be no glass or plastic between the light and the chameleon either. Sunshine is still the best source of light for them...of course. If you are keeping them indoors all the time, you may need to use supplements too. (And either way, gutloading your insects is advisable.)
You asked..."will putting a regular lighting fixture with a normal bulb for heat next to the pen be good enough for basking?"...I always put the light I use for heat/basking above the cage....be careful that its not close enough to burn the chameleon.
Can't help you with the free-ranging...I've never tried it.
You asked..."are asian pitcher plants toxic to veils? Can veils munch on the leaves without distroying themselves or the plant too much?"..I have no idea if they are toxic or not. I have seen a veiled completely defoliate a pothos plant...so if it chose to, it could finish off a pitcher plant too. Whatever plants you use, make sure that they have been well washed...both sides of the leaves. I alway make sure I cover the soil with something (pebbles that are too big to ingest) to stop the chameleon from eating the dirt too. Some dirts can cause impactions...as can substrates (which is why I also recommend no substrates for veileds too).
You asked..."what is the policy for handling veils?"...I handle them very little as a rule. Some will TOLERATE more handling than others....but there's always that silent stress factor to consider. Just because they don't appear to be stressed, doesn't mean that they aren't.
You said that some care sheets say they will eventually tame to limited handling, in my experience, once a grump always a grump...and even the ones that are not grumpy can become grumpy. Not saying that none will ever tame down...its all an individual thing. You asked if infrequent, gentle handling is ok...generally it is if the chameleon "willing". I have had some that would bite me every time if they could and I have had others that come out on me willingly. The willing ones even come over to the door of the cage when they see me...but be aware that it might be that they want out of the cage and not that they really want to be handled.
Here are three sites that have good information that you might want to look at...
http://www.chameleonjournals.com/vet/
http://adcham.com/
http://www.chameleonnews.com/index.html
Hope this helps!