Why splitting LCS across two hulls reduced usable fleet strength

How significant was the decision to build two LCS designs if each class was still expected to contain about 15 ships?

Very significant, because the program needed to replace more than 50 Perry-class frigates plus mine-warfare ships, and dividing that demand between incompatible hulls reduced both economies of scale and operational availability.

The proposed replacement requirement was closer to 50 or 60 ships once Perry frigates and mine-warfare duties were considered. Using a rough two-in-five rule, one 50-ship class would supply about 20 operational ships and perhaps another 15 available from vessels working up or returning. Two 25-ship groups would still provide about 20 operational ships in total, but the estimate allows fewer additional ships to be drawn from their separate readiness cycles. The illustrative comparison is roughly 35 available ships against 28, rather than a measured availability result for the actual LCS fleet. A common design also permits larger component orders, which can lower unit costs and attract more suppliers.