you cna have sencond dental and vision insurance, we have same thing as you, but we have just one medical insurance. that is enough.
if you have two dental insurance, you dont need to apy anything for almost any dental work, the two insurance will pay together. However, if you have two medical insurance, you have to pay two yearly contributions if your company can not pay 100% insurance (mostly you have to pay some percent monthly contributions) and if go to see Dr. only one insurance will pay. and you just pay co-pay. So another insurance useless.