I've got news! With 4.19, the first modeset will still disable the second
monitor,  including when going into X (without mode change), but after changing
the resolution to something else and back again, it now comes up reliably. This
is much better than before. Also, I get this in dmesg

[drm] dce110_link_encoder_construct: Failed to get encoder_cap_info from VBIOS
with error code 4!

on startup.

I don't know if this helps, and I'd be still willing to help debug given
instructions.