Does it look bad if I apply to two very different fields for grad school? I'm going to be applying to grad school (a combination of direct PhD and research thesis masters programs) this fall, and I cannot make up my mind between two different fields - machine learning (essentially artificial intelligence) and computer architecture (essentially processor design). They would fall under either the Computer Science or Electrical/Computer Engineering departments.

I obviously need reference letters to apply and I'm going to be asking professors I've done research with in the past. However, I'm worried that asking for reference letters for two very different fields reflects poorly on me; it makes it look like I'm not entirely dedicated towards either. 

I'm afraid that the letters I get from the professors will be a lot weaker than if I only applied to one field, because in that case it would look like I'm extremely passionate about it, compared to now where it looks like I'm not entirely sure what I want to do.

I'm wondering if this is a valid concern, and if so, how I should best address it. I could obviously pick one field now, but I'd prefer to put it off till after I get offers, so I have more time to make up my mind.

Just for context, out of the three professors I'm going to ask, one of the professors worked with me on a machine learning project, one on a computer architecture project and the last one on a signal processing project (not directly related to machine learning or computer architecture).
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!