NiMH battery at 660mV, is it trash or good?
Standard NiMH batteries have fairly high leakage and will lose most of their charge over a few months. However leakage reduces as voltage drops, so even after a year it should still have some voltage left. Individual cells in a pack may leak at different rates as well as having slightly different capacities, so over time their voltages will diverge. 

In my experience a 'new' NiMH cell that reads over 0.5V is usually OK. However it will probably need 3-5 charge-discharge cycles to reach full capacity, whereas if your other cell has already had some cycles it will initially have higher capacity. 

The problem you have is how to bring both cells up to full capacity when your charger only works with pairs. The answer is to charge them in pairs, but discharge each one individually. That way the lower capacity cell can't be damaged by over-discharge.