I performed 2 separate upholstery services. The client withheld final payment on the second service due to concerns with the first(which was paid if full-after it was completed) Both were on separate estimates. Both were signed by the customer. Would this be a breech of mutual obligation on the customer's behalf?

I am not a lawyer but I think the customer could not withheld the payment for the second service because of some flaw with the first service. In the first place, the first service was already completed. Second, the payment withheld was from the second service which is entirely different from the second. If I were you, I would consult a lawyer.
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When was the first service performed and paid for? When was the second service performed and paid for? The dates are important; they should have expressed their concern with the first service prior to having you perform the second... unless both services were scheduled on the same day.

I agree that the timing is a key issue here. It would be within the customer's rights to withhold payment if the timing is right.