Is it acceptable to ask my professor for help for another class during her office hours? I am in two classes (1 and 2), and my 1 professor never answers emails or holds office hours now that we're online. There is also no TA. Oh well, there's nothing I can do except include this information in my review at the end of the class.

I happen to know that my 2 professor has taught 1 sections in the recent past, and she attends her office hours without fail. If I give precedence to any student hoping to learn 2 during my professor's office hours, then is it acceptable to attend with the intention of getting help for my 1 class?
This might depend on local custom, but I don't think there is any reason that you shouldn't ask for help. You may be sent to the other professor or just told that the one you ask hasn't the time for you. But asking is fine. 

Of course it is also appropriate to say why you ask the one and not the other. But having a relationship with a professor makes it easier to discuss things with than with someone more distant. 

The professor will set boundaries if necessary. You don't need to be overly concerned with whether you are interfering with other students.