If two existing products manufactured in USA, with a US brand, that are unrelated and assembled (assembled requiring some machining and hardware) and utilized for a completely different application other than their original purpose,be constructed and introduced to market without infringement? If so, suppose those same two products were manufactured in China? Is there any way to possibly obtain a waiver from the US manufacturer since it will not compete plus the manufacturer will gain the benefit of higher sales volume?
If you are actually buying the products from the manufacturer that owns the IP, I doubt you have an issue with infringement. That said, it is quite possible that the manufacturer would not allow the use of their product for an unintended purpose. This might be quite reasonable if, for example, there were a risk of injuring someone it would expose them to potential liability. You best bet, is to simply ask the manufacturers whether they are willing to sell to you. Since your product entails additional sales they very well might be willing to work with you.
Just because the manufacturer is US based doesn't mean they don't also have patents in other countries so manufacturing their products in other countries does risk infringing on patents. To know if this is an issue really requires a legal opinion which you can't get from posting abstract questions on the internet.
The novel combination of two items can conceivably infringe a patent belonging to a third party with no relationship to the manufacturer(s) of the two items. Generally, a seller of a product has no control of the weird use you put it do unless there is some license involved (like software) or if the product is leased. If the products are lawfully sold in the U.S. it does not matter where they were made. If a "waiver" does come into the picture, it would technically be a license. If there is a very low likelihood what you are doing will take bread out of someone else's mouth, many entrepreneurs would proceed and address these issues if and when they were successful enough to be noticed.