If you want to use your uni-polar as a 'true' uni-polar motor then you need to use another hardware driver. The L293D contains 'half H Bridges' but what you actually need is effectively' a quarter' - known as a 'Driver Array'. The ULN2003AN is a good choice. With the center tap connect to +ve supply you can make the current flow in the coil by connect the 'a' terminal of the coil to ground. To make it flow the other way then connect 'b' to ground. If both 'a' and 'b' then no current flows through the coil, and if both are low then the current flows both ways - which effectively cancels each other out. So we can do this by connecting one driver to the 'a' terminal and another to the 'b' terminal. So two drivers per coil makes four per motor. The ULN2003AN contains 7 drivers - so not quite enough to drive two motors from the one chip!!