Ha, this exercise OWNED me. I grabbed my usual 12-15lbs that I like to do for lateral/front raises and there was no way I could finish even one set. I ended up using 8lbs.
It takes a little concentration. BUT, you hold a front raise while you do lat raises. THEN, stop the lat raises and hold. NEXT, do front raises with the arm that was holding the front raise. You then do the same thing other side. Hold a front raise and do lat raises. THEN, hold the lat raise and do front raise.
10 reps all 4 ways. Don't be alarmed if smoke literally starts coming off your shoulders.... JK. But it's pretty intense! I did this 3x through.