What is the validity of hiring transport for a known load at a per-unit rate, contingent on transporting another unknown load at a proportional rate?

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If both parties know the second load being transported (the one in the house) by sight, the contract is valid for both loads as they are considered one load. If only one party knows the second load, the contract is valid for the first known load but void for the second unknown load, as it constitutes two separate contracts, one valid and one void.