How do colleges know if you're doing multiple degrees at the same time ?

[mednat]

The short answer is they probably won't. The longer answer is most schools have an honor code and this would typically be handled by that code. The punishment for getting caught could go up to degree revocation in most cases. I'm not sure what the benefit would be to conceal this information, a second degree? If this is for undergrad, I don't the added value is worth the added risk.

With that said, most schools have a contingency for a second degree, and the requirements are usually around 30 credits in the new area of study. Meaning you don't have to be dishonest things just need to be done in an exacting order. Typically you need the credits to be issued after the first degree is conferred. I had intended on taking getting an undergrad in psych (for fun) after my business undergrad. Since I knew this up front I purposefully took no psych classes (even intro) for my first undergrad.

I would say it may be worth backing up a bit. What is your ultimate goal? Two undergrad degrees? If so, i'd suggest you find out the requirements at your school for two degrees and instead of doing them simultaneously, do them one after another. The rules are lenient enough to not require breaking.