Balancing an internet startup and a tenure track professorship
Nothing is impossible, but how many hours a week do you want to work? Do you have any life beyond these two endeavors. I don't participate in a startup, but at various times have published books and software and built interesting tools for students and professionals. 

But the warning here is that beginning a tenure track (especially) is a very time consuming proposition, especially if you want to actually attain tenure in a few years. You need to serve the students well at what ever level you find them. You need to establish an appropriate publication (research) history as well as a recognizable place in the profession through conferences and such. You need to have, or create, contacts with other researchers to help you in your own. This might come at your own University, given a large enough faculty with good synergy, but you may need to expand your contacts outside. 

In the US, the typical "slog" to tenure is seven years and you don't get do-overs. Not every paper you submit for publication is necessarily going to be accepted. 

You can read other questions on this site to learn some of the other problems that junior faculty have in adapting to the academic life. Not much of it is likely to come easily to you if you are really new at it. Good things happen in Academia, but they don't happen on auto-pilot. 

Some institutions have a history of tenuring most candidates. Some, alternatively, have a history of tenuring nearly no one, though some of the "failed" candidates have a good chance of success in moving, due to the reputation of the university. 

All that said, many (most) junior faculty do have outside interests of various sorts that take some amount of time and effort. It isn't impossible to succeed at both, but you may need to set parameters for yourself and decide early on what is the most important thing in your life. If you sort of stumble along at both, you aren't likely to be a success at either. 

Finally, what you learn and what you do in the startup may be a plus when it comes to tenure, though you indicate it is a different field, lessening the possibility. But if entrepreneurship is valued in your field it may be a help.