According to the official document, it is possible.
When you plan which pools to pair, you must keep in mind that only the following pairings are supported:
①Enterprise Edition pools can be paired only with other Enterprise Edition pools. Similarly, Standard Edition pools can be paired only with other Standard Edition pools.
②Physical pools can be paired only with other physical pools. Similarly, virtual pools can be paired only with other virtual pools.
③Pools that are paired together must be running the same base operating system.
In my experience, configuring pool paring between two same version pools is better. The reason is Skype for Business server 2015 and Skype for Business sever 2019 have some differences of feature and functionality. They cannot totally support all the features of each other. For more details, you could refer to this article.
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