Let's say I take 10 levels of Bard and then 10 levels of Rogue.
This would give me the following skill proficiencies:
- 2 from my background
- 3 from Bard level 1
- 1 from Rogue level 1
This is a total of 6 skill proficiencies, and with Bard level 3 and level 10, and Rogue level 1 and level 6, I would have 8 skills to put Expertise into. I can put one of these Expertises into Thieves' Tools, but there is still one Expertise left to be used.
Is this Expertise simply lost? Must I take one of the skill feats if I want to be able to use this Expertise?
Best Answer
Yes, the way you did it, you just lose this expertise.
Okay, let's start with some background.
Do expertise bonus stack?
It seems they don't. But what doesn't stack is their bonus.
BUT...
RAW, it seems you can put your expertise twice in the same skill. It just won't stack the bonus (therefore there is no reason to do it). Let's check why I say we can do it:
From Rogue
From Bard
Note that in the 1st Rogue level and 3rd Bard level, there is no "another" or "more" or "other" skills. Obviously, because so far they aren't considering Multiclassing (as it is a specific rule). You would expect to see some clarification about this in the Multiclassing section, but there is none.
RAW, you can get expertise twice in the same Skill, but it does not stack.
So yes, you just "lose" this expertise.
Quick Remark about the Multiclassing you did
Note that if you go the other way around multiclassing, starting with rogue, you will have 7 skill proficiencies (4 from rogue + 1 from bard + 2 from BG) and a thieves' tools, so you can actually use all your expertises.
Other solutions
A great comment added by Giuseppe added some more solutions:
TL;DR:
RAW, you lose 1 point of expertise. RAI, you probably lose 1 point of expertise. But you can still build your skill monkey by starting your multiclassing with Rogue instead of Bard. Or you can just talk to your DM.
Side note: Fun concept you got there to get to this question.