I definitely want this added. I keep running into servers not allowing it, wasting my time. Far too many servers not allow dual clienting, and it is necessary for normal gameplay if you want to vend(needs separate account), or have characters of different genders (also need separate account), or simply have too many characters for a single account.
Even if there are a lot of in-betweens, a simple Allow/Disallow option would help a lot.
It's not really important how you can or cannot bypass it, if it's not allowed, it's just better not to play there than risking a ban.
Afk timers could use their own entry. Even if you can't dual client, afk timers might be present to prevent leeching someone else for exp, so it is a separate issue.