what is the voltage across the diodes when placed across 5V, as forward biased and reverse biased
The voltages must add up to 5V, that is a constraint.

The current through each diode must be the same- they're in series.

If you use Shockley's equation for each diode and apply those constraints you should be able to solve it. 

Note that you can be sure that the voltage across the reverse biased diode is between -4.4V and -5V so the current should be pretty easy to approximate if you have a realistic idea of what Is should be.