This seems to be one of the silliest, yet easiest to fix limitations on Tumblr. Turns out to be that you can’t follow other people’s Tumblr blogs from your secondary Tumblr blog. This has been confirmed to be true by an Tumblr employee.
I guess that if you are considering to have niche Tumblrs you’d better open a different account for each, you can’t be expected to follow a religious blog from a weapons specialized tumblelog.
The thing gets even more serious if you consider that you can’t set up group Tumblr blogs as your main Tumblelog. Meaning that one of the main Tumblr functionalities, meaning following !, is unavailable to multiusers.
You might wanna check our list of tumblr follower checkers.