*In reply to rakesh reddy:*Except that according to the 1800-2012 LRM table 11-21, the length of a + b is max(L(a),L(b)) in a self-determined context. You need to write
ADDRESS : coverpoint 5'(tr.a+tr.b) {
// This should create 11 bins
bins low[] = {[0:10]};
// This should create 10 bins
bins med[] = {[11:20]};
}