I just answered this question within the last few months, but can't find the dupe.
The short version is this:
SO doesnt support personalized support requests. If it did, the experts would be constantly battered by specific requests for both difficult questions and trivial questions. It doesn't scale well, and those that contribute the most would eventually be driven away.
Second, your follow up question is as likely to be answered just as well by someone else as your original question was when you didn't know who might answer it. There's no need to specify a particular user just because they helped you in the past. Lots of other users are perfectly capable of answering a well defined, complete question.
Lastly, if you really want to get the attention of the person who answered you last time for your follow up question' it's easy to do so. Post your new question, then add a comment to their answer on your old question asking them to take a look at tour follow up question. If they visit the site and pay attention to their comment notifications, and most of the experts do, then they will get your message. They may even answer your follow up question if no one else has answered it by then.
But chances are good that someone else will beat them to it, and so much the better for you. Fast answers to good questions, isn't it great?