Does it look bad if I apply to two very different fields for grad school?
Based on the title of your question, I thought you were going to apply for two really different fields like French Poetry and Algebraic Topology!  So, while I appreciate the difference between software design and hardware design, I certainly would not characterise machine learning and computer architecture as "very different fields".  Both involve understanding and using computers, and I'm sure there are many people with an interest in both fields.  If anything, your interest in both fields reflects a holistic interest in the field of computer science and would probably be interpreted as a positive.

Regardless of how similar or different these fields are perceived to be, I cannot imagine that it would reflect badly on you to have an interest in both, and to be willing to pursue either field at graduate level.  While academia tends to push professors to become hyper-specialised, it is not expected that we will have worn you down to this point before even entering grad school!