1. Have at least inserter-capacity-2 researched.
2. Place two inserters.
3. Activate the "Override stack size" checkbox on both.
4. Give them different values for the override (one with "1" and one with "2")
5. Copy the inserters into a blueprint.
6. Open the blueprint parameterization menu. No need to touch anything in the parameterization menu, just confirm and resave the blueprint ([e] twice).