Can I take CS 429 and another upper division CS course concurrently?

No. Due to the difficulty of CS 429, the faculty are asking students to focus on their math requirements (calculus, M 340L, SSC 321, etc.) in this semester with CS 429. The only exception to this would be an FRI (CS 378) course. You would need to speak to the FRI department if you are not already involved in the FRI program to gain access to these courses.