Does adding layeers of sealant increase longevity
 

Many sealants tell you they last 6 months. They also say you can layer. So if yo put on 2 coats would you get 12 months?

You wouldn't get 12 months, maybe you would get 8 or so months by adding the second coat.

Product durability relies on the cross-linking bond a product makes with the paint, both sealant and coating bond / crosslink best with a 'bare' paint surface.

A sealant will of course bond with another layer, but the solvents used to help 'spreadability' will dissolve the layer it's applied too

Applying another layer will not extend the longevity, quite the opposite (see above)

The only reason to add a second coat would be to insure that you covered all the surfaces. As noted, the solvents in sealants will remove any existing sealant during application.

Rule of thumb, two thin coats.

The better approach would be to apply the sealant every few months, that will give you the best protection.