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

[Yanji]

Only speaking for TESU, they probably won't know- but you will have to sign a document stating that you've disclosed your credits and something to the effect of you not working on another degree at the same time- it's been a while, but I remember signing it when I filed for graduation. Signing something gives them the ability to revoke later, should they find out and decide to do so.

Yeah, this is good advice. When it comes to dealing with academic institutions, I would always err on the side of honesty and disclosure.