I think regardless of how it's done it has to be severly limited, especially in magic. Say only two casting skills allowed or something like that.

Training to only level 9 isn't feasable for a guild, leveling up past that point would be very difficult.

Combat I think would work out fine as it would only limit upper level fighter skills that IMHO should stay unique to fighters.

There was an AM spell, I can't recall what it was called, but it allowed people to temporarily gain the ability to cast spells from other guilds. Perhaps the polymath guild could be built off of a similar concept?

The way it worked to my understanding was the arch-mage would (I can't recall the name of the actual spell so I just called it 'steal spell'):

cast *6 steal spell at harold with ressurection

Then for the duration of that spell, which was several minutes, that person could cast ressurection. Harold would still be able to cast it without impairment.

It could be an interesting concept.. I wouldn't mind playing with designing such a guild.
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If you say plz because it is shorter than please, then I will say no because it is shorter then yes.