An Israeli company has developed software which it claims has the effect of turning a single core chip into a multi-core.

According to Mplicity's site here its product CoreUpGrade and is independent of processor architecture.

As an example it takes a single-core ARC625D from ARC capable of performing at a 270-MHz clock frequency and transformed it into a dual-core processor capable of performing at 237-MHz clock frequency across the two cores. The "dualised" ARC625D is a cycle-by-cycle compatible component which can be integrated with standard EDA tools and fab processes.

 
The start-up is aiming its product at computer intensive market segments that use large amounts of repetitive logic or face footprint or system power constraints.

More here.