There are several things that need to be considered when trying to make a homebrew rule like this. I guess because of the nature of what I am doing that things are going to get broken a bit, but unbreaking them is something that will just have to come on a case by case basis.
When it comes to armor and weapons you are looking at several things that need to be balanced. The most important element in my mind has got to be the cost - this will either make items totally unaccessible or game breakingly powerful. GP cost is the great balancing engine of 4e item management (unless we are talking essentials, and I am not).
Level - another very important element to consider for combining any two items or any three for that matter. Putting two properties of vastly different levels together could be very broken if not managed correctly.
Type - For both armor and weapons, if there are type restrictions for an enchantment, it is going to have to carry over to both enchantments.
Enhancement - where do the flat bonuses go?
Properties and Powers - Probably the reason you are going to be taking two or more different enhancements on the same weapon. They are going to need to be balanced in some way.
Well, these are the basic things I am taking into consideration for this homebrew rule of mine. Anybody think I missed something?