RTC capacitor with long lifetime selection
Placing to capacitors in parallel will have mainly two effects:


lower effective series resistance
increases capacity


It may increase the lifetime of the capacitor if the main stress (and cause of failure) is because of ripple currents. But generally it won't increase the lifetime as both parts are "living" in parallel, each using their lifespan. Twins don't have the double lifespan of a normal human either.

The lifetime of an electrolytic capacitor is often limited because of the electrolyte drying out. The higher the temperature the faster it will dry out.

I guess the capacitor you found was one rated for 2000hrs @ 85°C. I wasn't able to quickly find a supercap with a rated lifetime of more than that. So what are your options?


Review your requirements: 

How long is your RTC running without supply? -> do you really need 0.33F to bridge that time? Maybe you could use a smaller capacitor with longer lifetime? (10mF with 5000hrs @ 85°C is readily available)
Investigate other options: 

Maybe you can use a coin cell battery as backup? It stores a lot more energy than a capacitor, but you have to factor in self discharge and the temperature range might get further limited.
Can you place a temperature limit on your device? 

If the rated temperature of your product is 10K lower than the rated temperature of the capacitor, it's lifetime will roughly double. So if your device is usable only from -40°C to 75°C the lifetime of your capacitor will probably be 4000hrs. Note this is just a rough ballpark estimation and not a fact. Lifetime is a complex thing and if you have to be 100% sure you will have to do an in depth analysis using an approach with Arrhenius equation
Is the end of life of the capacitor actually that bad for your intended use?

Capacitors end of life is often specified by a loss of capacitance by 30% or an increase of the internal resistance by a factor of four. Now if you would use a bigger capacitor it would still have a capacitance of 0.33F after it's specified lifetime. So if you would use a 0.5F capacitor it would have 0.35F after 2000hrs and then it would last another 2000hrs to your end of life capacitance of 0.23F. The increase in internal resistance will make nearly no difference if you only draw a microampere from it.


A possibly related question, with good answers: How durable is a supercapacitor?