Is it alright to finish your online, second undergraduate degree while taking up law?

It's hardly possible. Full-time Law School is some 18 credit hours of lecture. Expect (no lie) another 3 hours of study/credit hour. Hence, a total of 72 hours.

There are 168 hours in every week.

For Law School, here's how it goes:
72 hours of lecture + study (this is 18 * 4)
56 hours of sleep (you'll need it)
14 hours of eating, commuting, shopping for food, exercising, paying your bills, and showering

You've got 26 hours left per week. And you won't even have that if you hit traffic, get sick, get into a relationship, or get a part-time job or internship.

Any school counselor worth his or her salt will tell you not to do this.