I had two rings, one increase the experience gain by 12% the other by 8%, is that mean my experience in increased by 20% or is it according to the highest item only?

Unfortunately I don't know the equation that is used for this calculation but I believe the increase is by 20%.

But maybe you could test it yourself. Remove both rings just before you complete a quest and see how much XP you gain. Reload your game, complete the quest again with both rings equipped and you should find the answer.

Items with +exp stack up. Thus, five items with +10% exp each will give you a +50% exp raise for each killed enemy in game.