Well I have three toons that I can level concurrently in theory but not in practice.
This is because my Mids can't get the same quests as my Albs. So I have to level them separately.
And after leveling so many to 50 in the last 10 years I think I'm getting tired faster than I used to.
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"Once more into the fray. Into the last good fight I’ll ever know. Live and die on this day. Live and die on this day."
- The Hunter's Poem