The anime features several classical tracks by composers such as Rachmaninoff or Beethoven.
Who are the performers behind the performance attributed to Kaori and Arima in the anime?
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There is a CD release (ESCL-4302) with all the non-original tracks featured in the anime. You can find more info about it on VGMDB.
As you can see there, the violinist is Yuna Shinohara (篠原悠那) and the pianists are Tomoki Sakata (阪田知樹) and Eriko Kawachi (河地恵理子).
VGMDB also provides scans of the booklet, here is the page showing this information (red rectangle):