Thanks for your answers.
This are the answers from t he costumer service. I post them here because others may have the same questions an will find this answers helpfull too:
Is the duplication working with having one SSD-drive set for cache. Is DrivePool writing the files to the cache SSD-drive and a duplication to the HDDs in the Archive?
Yes, but if you have duplication enabled, and don't have two SSDs, then yeah, it's falling back to using one of the HDDs.
How is the reading of this files, is DrivePool reading the files on the cache SSD-drive first if they are still there or will the duplication on the HDD slow it down?
if the files are duplicated, then it has a number of checks to optimize the reads, if read striping is enabled.
You can read about this here: https://stablebit.com/Support/DrivePool/2.X/Manual?Section=Performance Options#Read Striping
Would it be better to have two SSD enabled in the SSD Optimizer as SSDs to get for the files which are actually in the cach the duplication in the cache too, would this work?
If I use two SSDs in the cache how do I set them? Together in a subpool? Or adding them simple to the main pool and set them both as SSD?
Just add both to the pool, and enable them as "ssd"s in the SSD Optimizer balancers.
Would it maybe a better solution to get one SSD in the pool and not enable it as SSD-drive in the "SSD Optimizer" to use it as archive with file placement rules for my most used files?
Honestly, either works, but it depends on what you want in your setup.
If I try it like in 5. how is it with the duplication would DrivePool read the files from the fast SSD or the slower HDD? Maybe the slow HDD makes the SSD slow too?
It should generally use the SSD, unless it's busy, basically.
The nice think is DrivePool is working how I thought it should be working :-)
I ordered now another 1 TB SSD and will set both of my 1TB SSD in the optimizer as SSD. For my enabled duplication that should work best like the answer in question 3 says this is ideally. I will double my RAM to 16 GB too and keep my old CPU ans Mainboard.