Honestly, I have an itchy feeling about this, especially in a field like CS. There are those that will assume that a PI/advisor/supervisor did all the work when they appear in the list of "authors", rather than in an acknowledgement of other contributors. In math, and it was my practice in CS as well, to not consider any sort of authorship on my student's research output. And yes, they got some help from me, just as I did from my own doctoral advisor.
Having you as joint "first" author clouds the picture even further. If you want to emphasize that you were the advisor then let the student be sole first author (I won't suggest sole author, though it would be my practice).
Write an acknowledgements section in the paper that lists yourself as a contributor.
The difference for one publication between first and second authorship is so small as to not be worth polluting the space. And if the student objects to not being sole first author, then all the more reason to let it go. If you want to be a great advisor then boost your students.
There is something special about a student's first publication. Twenty years from now your generosity will be remembered. Forty years. Etc.