Close but no cigar. You named them correctly. The short version, Idle Mix Screw (the little one) is one turn Out, Idle Speed Screw (the big one) is one turn In.
A longer version. The idle mix screw is regulating the amount of gas the idle circuit carries. The more out the screw is the richer the mixture will be at idle. For an R90/6 the official starting place for the little idle mixture screw is one turn out. You may adjust this some but don't do that at first till you get the speed closer to what it needs to be. Set the idle mix at one turn out. This is the adjustment in the Bing manual for all R90/6es.
The idle speed screw provides a stop for the butterfly so it doesn't close all the way. It holds the butterfly open a very tiny amount. The two carbs are balanced with the idle speed screws. They need to be even in the sense that the two cylinders are running at the same speed and pulling the same amount. That is why it is called "balanced". Set the idle speed screws in one turn from the moment they first contact the bell crank of the throttle shaft. This is hard to do when the carbs are on the bike, the screw and the part it contacts are on the back side of the carb, so it's just hard to do. You want to try and set the speed screws to ONE TURN IN.
The Mix Screws ONE TURN OUT.
You need a method of balancing the carbs to get these settings even between the two sides. I'll ask again, Do you have the shorting rods or a manometer or other tool that compares carb vacuum so you can balance the carbs?