The second employer needs its own duty letter describing the actual proposed work, expected hours, supervision, and worksite, along with confirmation of how this role fits a listed profession. Reusing the first employer's letter or treating the second role as an informal add-on is not sufficient documentation.
Document the second role on its own merits
Ask the second employer for specifics: what work is performed, how many hours per week, who supervises the applicant, and where the work occurs. Gather any credential evidence the second role specifically requires, even if similar evidence was already used for the first employer. Keep a written schedule showing how time is divided between the two roles, since a reviewer may want to see that both are genuine, bounded professional engagements.
A Canadian citizen may apply for the classification at a US port of entry or preclearance location, presenting proof of citizenship, the employer's support letter, the degree and transcript and the applicable fee, or the employer may file Form I-129 in advance with USCIS. The employer's letter carries the case: it should identify the employer, state that the position is an Engineer position, describe the engineering duties in concrete terms, give the salary, state the intended period of up to three years and confirm the stay is temporary.