When I started out, I posted all my comic stuff on my main tumblr. After a little while I moved my comic updates to their own tumblr.
My reasoning for starting out on my main tumblr is that I wanted folks who followed me for my art to be able to see that I have a webcomic. I could reblog updates on my main tumblr, but then it might feel annoying or pointless to follow both? So I kept them together at first so that both my comic and my non-comic art could be a draw for my main blog.
My reasoning for switching to separate tumblrs was that, when I started posting more art on tumblr, I wanted folks who weren't interested in, say, undertale and deadly premonition fanart, to be able to still follow updates of my comic on tumblr. I reblog my ComicTumblr's update posts very occasionally on my main blog, but it tends to be more of a "hey, did you know I have a webcomic, you should check it out!" signal boost rather than an alert to every update-- I still encourage folks to follow the comic-tumblr if they want to get alerted to the weekly updates.
You could always have specific tags for each project, and let people know that if they like Project A but don't like Project B, they can use tumblr saviour to filter the tag "#ProjectB" and avoid those posts?