Only way to increase chance of drawing same server friend in Cup is to have very similar Q for T14 - but you need to get very lucky.

No idea what works with CL - but in custom leagues where my same level same server friends were the only friends there seemed to be a higher chance of drawing together (or a major coincidence occurred). eg Season 2 in my last OMA all 4 CL qualifying teams from 2 leagues (7 friends in each) made it to the same CL draw (and they all were in QF too )