I've been recently doing a little necromancy and I was dismayed to discover that it's not possible to unleash my inner Gandalf and use both a sword and staff at the same time, which got me thinking and I came up with a few thoughts on how to implement the possibility of using both at once.
Also, apologies in advance if this has already been brought up in the past.

I had two main ideas on how it might work: Firstly I thought of a kind of secondary system for certain weapons such as staves, for example when equipped in a secondary slot a staff can only be used for magic casts, and perhaps some very simple melee attacks. This allows for more freedom of customisation without (at least in my opinion) becoming too powerful and, Staves aside, you could make some fun combinations; Dual lutes anyone?

And secondly, I had the idea of a Dual Wielding skill tree which once learned allows for the use of two primary weapons but at a large disadvantage to either the second primary or both equipped weapons. Then as you level up the skill tree it eventually levels out the debuffs so that you no longer suffer when wielding two primaries.
Alternatively, the tree could remove the debuffs at 25 and then provide buffs onwards up to the max but I worry that might be a little difficult to balance out properly.

Anyway, that's my thoughts and suggestions. Hoping to be able to become evil Gandalf eventually.
Thanks for reading.