Is it possible to have more than one active artifact in Redemption?
Yes, you may have two Artifacts active.

Solomon's Temple's rules text seems pretty clear on the matter:


  Holder may activate a second Artifact from Artifact pile or hand on this Fortress if that Artifact was found in Solomon's Temple. Artifact takes immediate effect.


The rules for Artifact Activation only prevent one artifact from being active on the artifact pile.


  Artifact Activation: Only one artifact in the artifact pile may be active at a time. Once per turn, you may select one artifact to be activated and flip it face up on your artifact pile.