Also RAM kits are tested to work together
No they're not - not any more. 10 or so years ago they were individually tested and matched in pairs based on their electrical characteristics to ensure compatibility.
But 4 things happened in the times since:
(1) The purity and consistency of the raw materials used to make memory semiconductor devices has vastly improved over the last 10 years.
(2) The manufacturing and assembly techniques for RAM has also vastly improved, thus ensuring each module coming off the line meets much tighter tolerances, and therefore is as near identical to the next module as Man can make.
(3) The individual testing was not only expensive and time consuming, but (thanks to 1 and 2) testing revealed virtually ALL the samples worked perfectly and were already well within specified tolerances for pairing or trips.
(4) Makers of motherboards, chipsets and memory management modules vastly improved their products' abilities to manage RAM in dual and triple channel configurations. So even if a slightly mismatched pair was installed, the motherboard would still get them to play well together. There really is no reason, on a modern motherboard, why two RAM modules with the same specs, but from different makers cannot be used together in a dual or triple-channel setup and experience no problems. In fact, many motherboards allow you to use different specs, and it will sort out the best configuration for you.
The packaging of RAM in "kits" today is simply for convenience - to ensure you have sticks with the same specs. It is actually a HUGE win for the RAM makers as it saves big money on packaging, inventories, and shipping and whatever. And the consumer benefits too as shopping is easier and hopefully, a 2 or 3-pack is cheaper than 2 or 3 packs of singles. Make no mistake. If you have a triple-channel motherboard, you can buy a dual pack and a single pack of RAM with the same specs and expect them work together fine.
As for the slots on your board for dual channel, check your motherboard manual carefully, and ensure you have the right revision too. I have seen Rev1 use slots 1 and 3, then Rev2 of the same model number use 1 and 2. Caused some real confusion and frustration until the difference in the board revision numbers was noticed.