I feel like Mowen and Grenoldt are to be judged on a different standard to "serious" considerations of identities across soul-shards.
If we had counterpart shops which served the same functional purpose as Rowena's collectibles and Gerolt's forge, sure we'd be having some people going "oh they must be fragments of the same soul - Ardbert proves they don't have to look the same as their counterparts" and you could counter-argue that it's not necessarily the case just because they have the same job.
But here, they do look alike and I'm sure it's to be unsubtle about it. Of course it takes a lot of contrivance for a particular soul to be born looking exactly the same as a counterpart soul on another shard at the right time to both be at the same age at this moment... but this is fiction and the writers can make that contrivance if they want to.
If they did it for other important characters and we encountered doubles of other people we knew, then I'd be whining about implausibility. But here I can accept these very utility-based characters being counterpart souls "just because".
