Your subject line is a bit confusing, as batch split is more related to outbound deliveries, where you have not enough quantity of a single batch and need to add a second (or more) batch to get the total quantity.
I understand that you manage this product as 2 materials while your vendor manages it as one material .
I do not see any chance that SAP could automatically handle this.
There is no place to maintain somewhere a single lot size for 2 materials.