Aircraft manufacturers write SOPs/FCOMs with the lowest common denominator in mind. Issues like single engine taxi are reluctantly added because of demand, not because the manufacturer endorses this practice. Airlines with multi-manufacturer fleets will often try to make SOPs more similar where possible. Regulators will typically require the Airline to get an NTO letter (no technical objection) from the manufacturer before allowing an Airline to modify the basic FCOM/SOPs...