The easiest way to avoid having to have bars projecting through the formwork is to use a mechanical system that is in two parts.  The simplest such system for starter bars is the screwed bar.  An extension with a female thread is swaged onto a bar, which is then cast-in the concrete.  A corresponding male thread is cut into the second bar.  To avoid loss of cross section, the end of the bar is ‘thickened’ before the thread is cut. 

These simple couplers can only be used with straight or nearly straight bars.  However, by using a pair of female bars and a double ended male couple with a left-handed and right-handed thread, it is possible to connect bars with bends which could not otherwise be screwed-in due to clashing with each other.

A simple variation on single threaded bars uses metal formers to create voids in the concrete.  Special bars then click into these before the second pour takes place.  This system has the advantage of being able to work in shallow members such as floor slabs with metal decking to form upstands etc.

The mechanical systems above rely on the second bars being introduced after the formwork is stripped.  An alternative system has the starter bars contained within a ‘box’ that is cast into the first element.  After stripping, the bars are bent out of the box to form projecting bars.  
This system is supplied by a number of companies, and many variations are available in both size and number of bars.