how to listen in mono while mixing in stereo?

[sunnydyz]

i've heard a lot of you guys are suggesting to listen mono while mixing for making better sounding mixes. i'm using software stuff in most cases cubase . what i tried is to create a new mono output buss, but then i assign left or right outputs of my soundcard to it sound goes only from left or right speaker. how to make both speakers play mono sound mixed from 50% left and 50% right. the only way i can do this is exporting. but it is a little bit uncomfortable. i'm sure there's a better way of listening mono while mixing. how you are doing this? just want to try this mixing in stereo and checking in mono technique and save some time by avoiding export process