I think it depends if your going through a leaflet distribution company you're given a mix of leaflets to deliver at once so carrying the extra would be a struggle as your timed and often GPS tracked. Businesses also pay the delivery rate depending on the type of competition they are being delivered with and often pay a premium for a solus delivery.
If you really wanted to do multiples at the same time I would ask around your local takeaways and businesses rather than through a company :)