Fairly distribution of shipments among service providers

Summary:

We are experiencing an issue where shipments are not failry distributed among service providers.
We have 5 equipments for each carriers and per day capacity limits for each equipments.
Capacity limits vary for both carriers.
Cost is same for both service providers but still when shipments are created, priority is given only to one service provider, when its capacity is full then shipments are assigned to another service provider.

We want both carriers should have approx equal percentage of capacity limits utilized per day.
For example - Service provider A - utilizes 70% of equipment A capacity by assigning that many shipments to this carrier then service provider B should also utilize 70% of same equipment.