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. There is no fixed period, since it depends on citizenship, location, and how quickly the second employer's documentation is ready.
Sequence the second role around confirmed authorization
Set a milestone for receiving the second employer's complete duty letter, a separate milestone for the compatibility review against the first role, and a final milestone for confirming authorization before work starts. Avoid agreeing to a start date with the second employer until authorization is actually confirmed. Keep the first employer informed only as appropriate, and avoid any change to the first role's terms during this process.
A Canadian citizen can apply at a port of entry on the day of travel if the letter and credentials are complete, but the preparation — fixing the duty description, obtaining transcripts and resolving self-employment questions — typically takes several weeks. The sequence that causes problems is the reverse, where the equity is executed as part of a compensation package and the immigration question surfaces weeks later when a support letter is being drafted. Because admission is decided on arrival and the period granted is tied to the employment described, applying against a schedule that has not settled produces either a refusal or an authorization that no longer matches the work.