Mixing vaccines is promoted to muddy the waters, and further limit any liability to vaccine injury/death.

The vaccine makers get full indemnity from any accountability or financial loss from their quack medicine.

However they would encourage you to mix vaccines so that if for some reason the political climate changes and they are held accountable. It will help their lawyers very much if you would get injections from multiple pharma companies.

Therefore they can claim there's no evidence that the death/injury came from their product. It was the other guy's vaccine.