AdWords used to have a very explicit policy against double serving ads, until a couple of years ago when they introduced their ‘new’ help center with much shorter policy descriptions, apparently to make their policies easier to read.
That does not mean the policy changed, it’s just less explicit on their published help center. They replaced all of the explicit policies with fewer more general statements like “abuse of the ad network”, and “gaining an unfair advantage”, which presumably is the point of double serving ads.
So can you do it?
Well, you can if you follow their now unwritten policies.
If I recall correctly it used to say that you could have 2 separate businesses targeting the same keywords, as long as the product or services were packaged differently enough to justify 2 distinctively different offers for their users to purchase. However, selling the exact same product, packaged and sold the exact same way, will not likely pass muster.
What would work is one website that sells only kits or bulk orders, while the other website sells individual pieces. The “offers”, or the way they are “packaged”, must be different in some way to avoid the risk of violating the policy against double serving ads to gain an unfair advantage.
If you run the campaigns for both websites from the same AdWords account, Google will only serve one ad at a time, this will eliminate the risk of being dinged for double serving ads, but it also prevents you from double serving ads. So if that was your goal all along then you are going to have to be more clever than that and offer up something truly unique for your second offer and create a separate business entity to be safe.