In looking at the coverage/overlap between two carries, simple divide the coverage by the overlap to determine the ratio which defines how much the airlines overlap with each other versus how well their networks complement each other. Higher ratio is better.
Example. Airlines A, B and C each have networks can individually support 40% of a program. When paring A & B together we see 60% coverage and 20% overlap. Thus a 3:1 ratio. When pairing A&C together we see 65% coverage and 15% overlap This is a 4.3:1 ratio and means the A&C combination increases coverage versus overlap at a better rate than the A&B pairing.