I think you are wise to make a distinction. Computer Science and some related subfields use conferences as the major way to disseminate new results with journals more or less reserved for more settled ideas.
I can only speak for ACM (i.e. not IEEE), but many of the conferences are extremely important, SIGPLAN has several such and the SIGGRAPH conference usually introduces remarkable results. The Agile Software Development community holds a number of conferences with peer-reviewed work, but has no journal. The Software Patterns community works on a completely different model, publishes the work of conference-workshops, but again, no journal.
Part (maybe all) of the reason for this is the young (~75 years or a bit more: the Turing Machine was described in 1936) age of the discipline. Much of the development of the discipline has occurred in the internet age with frequent if informal communication.
Political Science, of course is a lot older (Sun Tzu, Machiavelli, ...) and more developed.
How to do it is another matter. If your work falls into the two disciplines separately it is easier - just have two sections in your CV. Different audiences will focus on one or the other and will likely know the expectations of the field they are interested in.
But a lot harder if your work is integrative of the two disciplines. It will be harder for each audience to appreciate the difference (A footnote that "Buffy sez" won't get you there, of course). I don't know of a reliable and official source for the things I've said here to which you could point, though there is probably some documentation of it "The importance of conferences to CS", say.
However, I think the issue is more important for a new member of the profession, rather than an established member as I suspect is the case of the OP. Your general reputation will be clear to people with no more than an introductory statement (among the rest) that you have published in appropriate and (respected) venues to the disparate fields.
With respect to grant applications, I think you should actually have more flexibility to inform reviewers than in a CV. If the granting agency is more concerned with one field of the two or more you are covering, then they will be mostly concerned with your references in that field and wouldn't know much about the others in any case.
But you could give as part of your background, I hope, in most cases, an explanation of the importance of conferences in CS re journals. It would only take a short paragraph and a reference or two. However, I don't think it would really be critical unless the grant was primarily NOT CS but the reviewers had to understand how CS actually works in practice. But I think in such a case (never actually having danced on the cusp, here) that they are more interested in the "other" stuff and see the CS part as primarily methodology. But you need to be able to back up your methodology in any case. The CS folk will "get it" and the others will think the "other stuff" is the most important anyway.
I find it hard to believe that a political-science grounded grant would fail for reviewers thinking you weren't a mover-shaker in CS. What I mean by this is as follows. If I'm a political-science guru reviewing your grant application to explore some deep topic, I'm totally focused on my own field and on what I think is your potential contributions to it. The fact that your methodology uses, say, machine learning, will be foreign to me and it will be hard for me to judge in any case. But I'll be intrigued. However, if I think you are primarily interested in CS and are naive about political-science, I vote no. But if you are clearly well published in the field I'm interested in, and your arguments are good I'll vote yes without a deep dive into your position in CS, though I will need "sufficient assurance" there. The one overbalances the other. I suspect I'd be intrigued by a new approach to things I consider important.
However, if I'm an AI guru instead, I understand the quality of your CS publications and will focus on that. I'll see the political-science stuff as an intriguing application of what I consider important. But here, I won't know about the quality of the political-science journals you are published in without asking someone.
The importance of journals v conferences is asymmetric, but the problem you face is, itself, asymmetric in a matching way. Pretty sure, anyway.