A product cannot be active at more than one location if one of them is a fulfillment service location.

For example: 

The multiple locations don't work because the limitation is on the fulfillment location. In Shopify, there is a difference in inventory location and fulfillment location.

SKU 123 already has one fulfillment service location Simplify and that SKU is not able to be set to another location Simplify(123). 

It's a restriction from Shopify that we can't bypass it.

So, one SKU can only have one fulfillment location.