External monitors show blank screen until restart MacBook Pro (15-inch, 2016) with 2 USB-C to HDMI adapters. The setup works great if I start up in that configuration. 

If I connect the adapters while the machine is running or simply sleeping, one of the monitors will never get a signal and stay in low power mode. The OS sees the monitor, it shows in the display arrangement settings and I can move the mouse into it, but until I restart, I can't see anything on it.

Once I have restarted I can let the machine sleep or whatever, and the display still works unless I disconnect the adapter. As soon as I do that a restart is required to use it again. 

Using MacOS Sierra (10.12.4)

Using this adapter: https://www.amazon.com/gp/product/B01IUXA9YK/
The video signal on the  Thunderbolt connection on your MBP is actually Display Port so you are actually going from Display Port to HDMI.  I am not a fan of converting signals because it's flaky at best.

If you must connect to HDMI, try an active1 adapter like the TRENDnet USB-C to HDMI 4K.  They have two versions - one that does 30Hz and another that does 60Hz (it's the Power Delivery version).

What is preferable, is to go from USB-C to Display Port which is actually Display Port to Display Port, so no there's conversion involved.  No conversion means less points of failure.    

jksoegaard's comments provides more evidence of this:


  Just to add a real-life experience: I'm using 2 USB-C to DisplayPort adapters (which are basically similar to the HDMI adapters) - and this works without any problems in that regard. Several times a week I plugin in both monitors after the MacBook Pro is booted (basically I never turn it off) - removing the cables again also never give problems



1 I wrote an answer on this very topic:  Passive vs. Active Adapter