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Would it be possible to adjust vardiff for different workers individually ?
Difficulty when using Stratum is per connection not per worker. Bitminter will choose a difficulty for the connection that is suitable for the fastest worker using the connection. That can be a little heavy for workers that are considerably slower.
Setting up two Stratum connections, for instance with Stratum proxy, should work. I'm not sure why you are experiencing problems with that. If you want the difficulty on the old proxy that you had already to drop then you may have to restart it.
Thank for explanation, it works as expected,
when I've stopped ASIC miner, difficulty dropped down for all proxies/workers
and after restart of faster ASIC miner, difficulty rised up again for all proxies/workers
It's clear now.