I need to have two different email accounts running concurrently but can't find anything in the help articles

You do not need multiple profiles for what you plan to do. The two accounts can run in same profile.

To add an account: ≡ (menu at top right) > New account

Thank you both for your replies. If I've understood correctly I can have email accounts from two completely different hosts, with different servers, each of which is set up in a separate Thunderbird account.

I must admit I was thinking of having two separate Thunderbird installations so I could just switch between them on the Windows taskbar. How would this compare, for ease of access, with two accounts on one Thunderbird installation?

Yes, you can have many accounts with different addresses and servers in Thunderbird.

There are some significant differences in ease-of-use. Opening the second profile would take a few steps. Then switching between profiles would be like switching between apps. If your accounts are in one profile, you must open only one profile and you can switch among accounts with one mouse click.

If you have two profiles, you would have to repeat your customizations, e.g., folder and column layout, installation of add-ons, etc., in each profile.

If you ever want to copy or move messages between accounts, for example if someone writes to you using the wrong address, it is much easier if all the accounts are in one profile.

There would also be differences in ease-of-administation. For example, you would have to back up two profiles.

I would probably think of more differences if I thought longer about it.

Using multiple accounts in one profile is very easy. I don't know a good reason to put accounts in separate profiles.

Thank you Rick.

I see now that I don't need multiple profiles, so that's answered one of my questions. Since you say switching between accounts is one mouse click that seems to imply that only one account can be on screen at one time. I'd have quite liked to be able to have them on screen simultaneously which is why I visualised installing a second copy of Thunderbird (in a different location). However I realise that would involve setting up the profile in that copy to match the original and I like the idea that simply adding a second account in the original profile would preserve the customisations. It looks as if that would save a lot of work.

Since you say switching between accounts is one mouse click that seems to imply that only one account can be on screen at one time.

That is just one way. You can also open multiple windows open and see different accounts at the same time.