Ordinarily, back-to-back ticketing is disallowed by the airline, and may result in loss of any frequent flyer benefits, additional charges, and/or loss of rights to fly with them again (if they can't peg you with the first 2). If, by "multi-city ticketing", you mean what others refer to as end-on-end ticketing, then this is perfectly OK. End-on-end means that you buy multiple R/T tickets using an intermediate city or cities, but that all segments are flown in order.

Here's a good description of both types.