I think it’s best to simply email the alumni with your question, and phrase it something like “I hope you don’t mind that I’ve emailed you. I have another question and am wondering if you might be able to answer it…
I appreciate the time you’ve already taken.”
Assume the alumni is a busy person. Fewer emails are better. If the alumni minds, he/she will say sorry, but it’s not convenient. If they don’t mind, they will give you an answer, and probably say feel free to ask any other questions.