I know this is contrary to what @OldUser said and what he said is correct, you can pay with one check. The reason I paid with two is that if there is some defect with your I-765 submission and they have to reject the application, then they have to reject both the I-485 and the I765 because they cannot process just the I-485 with a check in the amount for both fees. They will reject it for "Incorrect Payment" and now you have to start over with both forms. If you have a separate check for each form, then if they have to reject the I-765, at least your I-485 will continue processing and you can just re-submit the I-765 once you get the NOA1 with receipt number for the I-485. Just me being overly cautious but I like to mitigate any risk of delay that is within my power to control, no matter how small the chance of it happening.