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You won’t need a letter.
When it is time to submit your work experience, you simply fill in what you’ve completed at each job. If you’d like to use your ex-boss as a referee, you can contact them then and ask. They’ll need to fill out a form that CFA Institute will provide, it’s pretty simple and self-explanatory for them.
And yes, you can include experience from multiple jobs.
Hope that helps, let me know if you have further questions!