if you want the best performance I'd go for one repository for each volume, and avoid any additional split. As we are writing a new paper about ReFS configurations, let me reuse a part of the text I wrote on this topic:
We suggest to use the available space in a single volume; if a server is configured with multiple repositories, one Veeam datamover per repository will be started, with its amount of consumed CPU and Memory. So, any calculation about server sizing needs to be adjusted, and performance is decreased as a datamover has to share CPU and memory of the same physical server among its siblings. By having one single datamover per physical server, performance and hardware usage are optimized.
In your case, it would be two volumes, but the general suggestion here is to go for large volumes, and have as few as possible.
EMEA Cloud Architect @ Veeam Software
Veeam VMCE #1