Best Practice when setting up a contract and you are billing the base to a lease company and overages direct to your customer, is to set up one contract only.
If you have two separate contracts, EA will force you to manually select/link the correct contract when entering a service call and order. If you have one contract for that equipment, it will automatically link it to the contract.
1. Contract is Set up in your Customers Name.
2. Under Billing Tab, set the Bill To with the Lease Company name.
3. Check the box 'Use alternative bill to for overage' and select your customer.