If? h°rxn and? s°rxn are both negative values, what drives the spontaneous reaction and in what direction at standard conditions?

The release of free energy drives the spontaneous reaction.

Spontaneity can be determined using the change in Gibbs free energy (the thermodynamic potencial):

delta G=delta H - T*delta S

where delta H is the enthalpy and delta S is the entropy.

The direction (the sign) of delta G depends of the changes of enthalpy and entropy. If delta G is negative then the process is spontaneous.

In our case, both delta H and delta S are negative values, the process as said is spontaneous which means that it may proceed in the forward direction.