A Ring of Enduring Arcana gives me CL+4 versus enemy dispel attempts (Complete Mage, p. 126):
You are considered four levels higher than your actual caster level whenever another character attempts to dispel or counter one of your spells.
Would wearing two of them give me CL+8 since as I can see, the bonus is untyped?
No. You would still only have CL+4, to achieve a maximum CL of 24.
The ring of enduring arcana specifically says
You are considered four levels higher than your actual caster level….
So while your effective caster level for some purposes is raised by four, your actual caster level remains unchanged no matter how many rings of enduring arcana you wear.
This means that, for example, if you have an actual caster level of 10, putting on one ring of enduring arcana causes your caster level for some purposes to be considered 4 levels higher than your actual level—therefore caster level 14—, and putting on a second or third or fourth ring of enduring arcana still causes your caster level to be considered 4 levels higher than your actual level—therefore continuing to be caster level 14.
