[Olli Kalma]

I disagree with WillowIX.
It's possible to take several bard guest at one time.

I train two character to bard doing next thing:

First I get bard training mission to both.
Then I go into serpent temple and searched scroll.
Casted create portal outside the temple ( inside didn't work! )
Casted Call to Home.
Finished mission by giving scroll to thieves guildmaster.
Go outside the town, casted Teleport.
Entered into temple again and retrieved the scroll.
Out from temple, Call to Home and into thieves guild.

The scroll appears next time when one of your character had gived it to thieves guildmaster.

If you didn't finished bard training, scroll won't appear until you finish it by first character.

Well like WillowIX said, it's better to get mission to one person, finish it and then get it to another person... But my point is that you can train two or more at the same time!

Don't know does Assassin's dagger effect work in this case too.. Might try that.
P.s. Read only if you want spoil your self!
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When you get Assassin's dagger, you can keep it by passing it to another person after thieves guildmaster ask do you want to become assassin?
Now you can keep dagger but become assassin! And can train several assassin!