Why Renown and Repulse could not immediately join American cruiser groups

Would it make more sense to combine Renown and Repulse with American heavy cruisers?

That could be a longer-term aim, but the immediate force needs ships already able to fight together, especially at night.

Renown and Repulse with American heavy cruisers and carrier support could form a powerful force short of the Japanese battle line. Effective cooperation would nevertheless require conversations, practice and compatible procedures, especially for night fighting. Integrating a carrier is considered easier than combining British and American gunships in close tactical operations. The immediate preference is therefore to assemble British fast and slow battle squadrons that can operate together at once. A carrier force would help them hold until American construction supplied reinforcements. Cunningham is proposed as a sufficiently senior commander for this exceptional concentration.