I'm not sure which enclosure you have, but a thought... If you double the roof thickness, it may actually chafe and not last as long. Might be better to use one layer, but hang on to that spare in case the first blows out.
When hurricanes were expected while I was building in the tent, I would also wrap the boat in another tarp, in the tent, in case the tent failed...
Here it was a very steep gradient high that blew in with violent gusts from the west (60 to 70 mph gusts), that blew out the second roof (cheap polytarp) that was then maybe one year old (the original roof lasted about four or five years with that sort of wind possible once every couple of weeks in the fall, winter and spring), and the boat was already in the inner *cathedral*. The original roof also held about two feet of heavy wet snow a couple of times! I had to sweep and rake that off very carefully. The stock roof, depending on manufacturer, can be very tough stuff, but you need to duct tape any small holes or splits immediately before they grow quickly.
The sides fail much sooner because of all their flapping. Be sure to use the bungies to secure the roof if that is what came with your tent, as they help vent, ease and distribute loads and greatly increase the life of the roof.
Earlier, before the cathedral was built, I just wrapped the hull well with a second set of tarps... and secured it to the cradle, and weighed that down with buckets of rocks and sand and other weights. Begin well, build well, finish soon and well.