A front facing cam is a rear facing cam if you mount on the back. Same difference.
The trouble with 2 cams and 1 recorder is the data rate. Whereas with a dual system with 2 cams and shared recording / processing chips, the power of these can restrict quality. If you fit 2 separate cams, then each has it's own supporting hardware and you guarantee yourself the performance of each separate camera. Plus another advantage, if one fails, you only have to replace one, not a dual system which generally costs twice as much as a similar single unit.
You can also tailor units to your needs. eg 2 x A119S's or an A119S upfront or an Opia 2, and a Mobius or WR1 rear, (very small cams) for a more discreet view, or any other combination of any manufacturers models or units. The choice really is yours if you use separate units.