if there's some kind of misunderstand on my side, please ignore. I've got the Steinberg UR22 running through ASIO4ALL, but this should work the same as with the Yamaha driver.
First, I think you should rig your mike through input 1 and the guitar through input 2 as there is the "Hi-Z" knob for input 2 for these kind of high impedance instruments. (Make sure to check the documentation about that.) Keyboards may run through either of the two inputs.
Then you adress two seperate audio tracks in Cakewalk and for your mike you choose the input" Left Steinberg UR22 1" and for your guitar" Right Steinberg UR22 1". You should get two seperated mono signals in Cakewalk. It's basically as the others said.