As others have pointed out, the fields that you've mentioned are in fact quite similar, and it is completely acceptable to apply for different fields. I applied to 4 different departments (actually even two different departments in the same grad school in some cases), as different as Biology and Informatics! I think it all comes down to how you handle your applications and recommendation letters.
If you're applying to grad schools in the US, this might be easier, as you can even address the fact that you're interested in both ML and Computer Architecture in your SOP, if the department has faculty with research in both of these fields (not necessarily the same professor; I mean it has professors working on ML, and professors working on Architecture). But you should handle this well. It is ok (at least in the US) not to be completely sure about what field you're going to end up doing your thesis on, since in Academia, interests are quite fluid and in fact the committee always knows that your interest might completely change and end up not working on any of these two as it is not uncommon at all (but that does not mean that what your write in your SOP does not matter, it just means they may not be as strict as you think about you being dedicated to your current field of interest). I can't tell you what is a good way to address both of your interests in your SOP, but I'm sure your professors could be a great help if you can go and ask them (and if not, you can ask this as a separate question and ask for advice from the community here in StackExchange).
If you are not applying to US schools, in many other countries (such as most EU countries), they expect you to have a relatively clear idea of what you are going to do before you start your Ph.D. In those cases, you may want to separate your applications into two disjoint groups and even if you want to mention both of your interests, you should use a language that puts more weight on one (I'd recommend you ask for a others' opinion about this case, as I know very little about it). You can also do this for the US schools if you are applying to a certain school and you know which field you'll be pursuing in that school.
The reason you may want to mention both of your interests even if one is not directly relevant to the program you're applying to is just for your SOP to match the recommendation letters you get. If you can't get 6 letters (to divide them into two groups, one for ML and one for Computer Architecture), then you should handle the language you use in your SOP carefully so your SOP matches both what your professors write in their LOR and your the field of research of the faculty at the department you're applying to and in particular your faculty of interest.
In any case, I think it is important that you show a draft of your SOP to the professors who are writing your LOR before they write it. You should also explain your interests to them (I think they should be fine with the fact that you are interested in two different fields, but I think it's better if you get the opinion of a professor here about that). That enables them to write a letter that leaves room for some flexibility for you in writing your SOP.
PS: I applied to departments in four different fields, and I got offers from 3, but I had only one offer from two of them, and all my other offers and even interview invitations were from the other one. I think that was because my SOPs (and probably also the LORs) ended up having a biased language in favor of the field I studied my undergrad in. Even though that field was (and is) also my preference for grad school, but I did not intend to write a biased SOP. So you would want to be careful in writing your SOPs and perhaps ask someone else to read your SOPs and give their opinion on whether you sound the way you intend to sound or not.