Can't run multiple servers at the same time

This is something that has changed this year, called the 'single sign-on' system. It's supposed to be an improvement but caused multiple issues when first introduced earlier around May.

It is still possible to play 2 (or more) accounts at the same time on different worlds.
If you get the direct link to a world it may be possible to play two worlds with the same account by opening one and then opening the other directly.

most browsers have a private or incognito mode - one way may be to log in with the standard window then login with a different account in a private mode.

I regularly manage to login with 2 accounts by using two different but similar browsers. I currently have Chrome and SRware's Iron browsers running with two accounts. I can login to the same worlds or to two different worlds and also use subs on each browser.

SRware's Iron browser is very similar to Chrome (without the issues lately) and can run the same scripts and extensions.

I user Waterfox (a Firefox 64-bit version) if I want another browser window on another world open. I usually don't bother with that because 2 is enough with their subs!