I work in a restaurant in the catering department where there are 2 people in that department and we split all tips 50/50. My boss is telling me that when one of us is not there that we are not allowed to split the tips 50/50, that 100% goes to the person that is there on that day. Is this legal?

I know absolutely nothing about law, but common sense dictates that if a person is not working on a given day, they do not get paid- hence no tip.
Now, if it were a job over several days and the tips were combined from those days, it might be a different story; the tips should in theory be split based on the days said person was present.

The law itself will depend on where you live. I found one bit of info you might find helpful from New York State: Section 196-d of the Labor Law provides as follows:
"No employer or his agent or an officer or agent of any corporation, or any other person shall demand or accept, directly or indirectly, any part ofthe gratuities, received by an employee, or retain any part of a gratuity or of any charge purported to be a gratuity for an employee. This provision shall not apply to the checking of hats, coats or other apparel. Nothing in this subdivision shall be construed as affecting the allowances from the minimum wage for gratuities in the amount determined in accordance with the provisions ofarticle nineteen ofthis chapter nor as affecting practices in connection with banquets and other special functions where a fixed percentage ofthe patron's bill is added for gratuities."

It has nothing to do with splitting tips, but it says that an employer can't take the gratuity for themselves; but there's nothing about how to split tips.

If you're in the US try this website: U.S. Department of Labor - Find It By Topic - Wages - Tips I couldn't find any info specific to splitting tips, but you might have better luck.

I'm curious to see what the lawyers say about this...

I agree. Now if you were the other worker and were complaining about having to share your tips with someone who didn't work, I would say there was a problem.

Or, if you were to say that the other worker got 100% when you weren't there, but you had to share when he wasn't there, again that would be a problem.

But requiring that you split when you both work, but each gets 100% when they work alone, that seems only fair and is certainly legal.