Yes, a student can have a 504 plan and an IEP.
For example, a student with an IEP comes to school needing a temporary medical plan, should receive a 504 plan. This would be a short term 504 plan for short term accommodations. A broken hand would require a student to have the accommodation of a scribe (someone to write for the student).
These instances are few and far, though.
In general, everything that is included in a 504 plan can be included in an IEP so there is not typically a need for both.