Wanted to Share this, since I was having same issue. Surface Book with dGPU, Microsoft Dock, 2 Microsoft Mini Display to VGA adapters feeding 2 PLANO 1920x1080 monitors.
Only One monitor would work at one time, I tried every combination of cable swapping in troubleshooting, If I unplugged the working monitor from dock, other monitor would immediately activate. Plug it back in, turn off the active one and activate the one I just plugged in. Very frustrated and reading a lot.
When I happened into Display Settings / Advanced display settings and looked at the multiple displays options, there was an option to "Duplicate desktop on 2/3" Odd since the setting only showed display 1 (surface Book) and 2 (the one working monitor). I applied this setting and wala, both monitors came to life, however useless as they were just mirrors.
Then checked display options again, and there was a new option "Extend desktop on 2/3". This option was not there before, but applied it and now both monitors are active and extended desktop as I wanted. Settings hold when unplugging the dock and reconnecting so far. No Idea why MS can't get this right out of the box. This is all their stuff, so no 3rd party excuses.
Hope this helps