Operating system: Windows-10-10.0.17134 64 Bits
Graphics card: GeForce GTX 970/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 382.05
Broken: version: 2.80 (sub 57), branch: blender2.7, commit date: 2019-04-17 19:26, hash: rBb46245470f79
Short description of error
When two (or more) 3D viewport panel exist, viewport overlay option for onion skin for one panel affects onion skin of other panels.
Exact steps for others to reproduce the error
Attached .blend file have two 3D viewport panels and left panel is set onion skin OFF, and right panel is set onion skin ON.
However, onion skin does not show in right panel.
When you turn onion skin ON in the left panel, onion skin appears on both panels.
I noticed that this bug happens only in draw mode or object mode.
In edit mode, onion skin is shown in right panel (and not shown in left panel).