Twin GNS 430 GPS on Carenado Piper Seneca V

[left]I've been going crazy for a few days to understand the purpose and use of the two GNS 430 mounted on the Seneca V Carenado. To fly with GPS, one is enough; I tried to fly with the traditional Vor system in Vor, thinking that I could set a route on GPS1 and the next on GPS 2 and then switch but the latter never follows the Vor and, instead of turning, he starts to spin the plane in the round. I have to TURN off the GPS 1 and then the 2 follows the tuned Vor. Is it possible that the two devices are only used as backups of each other in the event of a failure? Am I wrong? Help.
Thank you.
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