A container type for 2 singleton types.
init : a1 -> a2 -> Singletons2 a1 a2
Initialize a container type for 2 singleton types.
specs : (Ecs.Internal.SingletonSpec a1 (Singletons2 a1 a2) -> Ecs.Internal.SingletonSpec a2 (Singletons2 a1 a2) -> specs) -> specs
Create all singleton specifications for 2 singleton types.