Little tip for people using Mastodon's multicolumn advanced web interface:

You can extend the number of columns by clicking the slider icon in the top right corner of a column and selecting "pin".

There is no slider icon on profiles BUT you can pin particular accounts:

1. Create a new list
2. Add the account you want to the list
3. Pin the list

You will then have a column pinned which only contains posts by that account (and of course you can add more accounts to that list if you want).