I'm trying to make a very simple mod.

The idea is that the Magelight spell lets you have only one light at a time. If you cast another, the first fades out.

I'm trying to remove this limit, but I can't find it anywhere in the spell's properties.

Any idea where I would be able to control such a thing?

I cannot find anything either. It's possible that this is hardcoded into the "light" effect archetype.
You could try removing the Keyword from the effect, but I don't know if that'll help.

If it does not, you could try a script that casts a magelight spell with a different caster.