Connecting MacBook Pro to multiple monitors, one in portrait (vertical) mode I‘ve got a 2015 13“ MacBook Pro and want to connect 2 external monitors. One LG ultrawide (resolution 2160x1080) and a Full HD Monitor rotated to be vertically upright (resolution would be 1080x1920). 

My question is: will the Mac render a 1080x1920 image if I select resolution: 1920x1080 and set the rotation to 90 degrees?
Yes, simply go into System Preferences->Display.  A new window will pop up on your primary monitors, as well as your external monitors.  The external monitors should have a "Rotation" drop down menu, in which you can select "Standard", "90º", "180º", and "720º".  If you select "90º" and hit "Scaled" under the resolution option, you'll see that the standard width and height are all swapped, and that 1080x1920 will be the default option (but you shouldn't have to set it to scaled. Default for Display is probably the best option).