Sorry Unclwish I won't delay the release to investigate your bug since the Turtle fork is somewhat an emergency issue.
Ok-ok. I'll wait. Note that cards are both RX 580 8Gb. Now I forced to set 2nd GPU 1 thread and 16 multi_hash to start mining at 1st GPU and 2nd GPU almost free, but even 1x16 gives some load on 2nd GPU...