can i combine 2 module 2x4gb on ga-z87x-d3h

Solution
as long as they are the same kit and don't have any differences such as voltages, timings, or latency. You shouldn't have any problem as long as they are the same exact kit as you bought before.

Tradesman1

Legenda in Aeternum
If it doesn't work and it might not, (it's never a good idea to mix sets of DRAM, even the same exact model, it can be problematic. This is why there is such a large assortment of DRAM sets available, those sticks in a packaged set are tested to work together, you might pull 10 sticks right off the assembly line and not find 4 that will all play nice...If you get them and have problems, give me a shout and we can try some minor voltage and timing adjustments
 

lfkfkfkffs

Admirable
I would just go ahead and order them. Just run memtest86 once you receive them. If memtest86 picks up any errors, or if you notice any just return them through g.skill. Their rma and returns department is very fast and nice.
 

Tradesman1

Legenda in Aeternum
As mentioned, no guarantee in mixing sets - even the same exact model - ever wonder why a 4 stick set cost more than 2 two stick sets, it's because all the sticks in a package are tested to work together, you might pull 10 sticks right off the production line and only find 4 of the ten that will play together to make a set, or you might not even find 1 4 stick set out of the 10 sticks. The additional testing required is what raises the price. Also with sticks 1600 and up that are programmed for XMP, the programming is by the set i.e. in the advanced timings a 2x4GB package might require a tRFC of 208 while the same sticks in a 4stick package may require the tRFC to be 278