When physicians split up and one will be collecting the A/R for the other, what is a fair percentage for the collector to charge the other physician?

It really depends on the specialty and exactly what services are being provided. In this case, it sounds like it's only follow-up work (and not registration, charge entry, or coding), so you'd be on the low end of the range. The following are fee percentage ranges for various specialties, according to the Healthcare Billing and Management Association (previously the International Billing Association), a trade group of billing professionals: 