Do not assume authorization for the first arrangement covers different employment.
Check the actual next activity
Review employer, duties and current permission. A completed contract or new customer request does not itself authorize follow-on work.
Allow time to prepare the second employer's duty letter and evidence, confirm compatibility with the first role, and complete the correct authorization step before that work begins. Ask the employer to record the start date as subject to authorization, so a scheduling change does not become pressure to begin early. Record the delay, revised duties and expected completion, then compare them with the actual admission or status documents. For a Canadian, a fresh port-of-entry presentation may be appropriate, or an employer may use Form I-129; neither permits altered work before authorization. Admission is granted for a period not exceeding three years and is tied to the employment described, so applying before the structure is settled produces an authorization that does not match the work. The actual offer, credentials and applicable visa or admission steps remain relevant before U.S. work starts.