Double vs Single Condenser Lens

I've started taking 6x6 photos. Instead of buying a new enlarger, I'm
adapting my trustworthy MPP condenser to take 6x6 negs. This has
involved fabricating a new lens board, fabricating a new neg holder,
purchasing a new enlarging lens and replacing the condenser.

Now, I have a pair of spherical condenser lenses but unfortunately I
have no cell to mount them in. A single lens on its own fits snuggly in
the enlarger head and seems (by eyeball) to give a uniform illumination
across the easel.

I know some enlargers use single apsherical lenses but what
advantage/disadvantage do I have using a single spherical lens?

Come to think of it, I'm not to sure why condenser lenses come in pairs!


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I need more time!