They can be doubled up easily to offer better protection, but they won't last more than 1 or 2 uses. I just assumed they would be hemmed around the edges. That would make all the difference. I have requested refund. We will see how it goes.
Try taking a colander like the one used to drain the water from cooked spaghetti, then put a 2nd colander under the first one before you pour the spaghetti from the pot. You're gonna get pretty much the same amount of water getting all the way through, with the only difference the insignificant amount that clings to the surface and the noodles.
Anything you use as a filtration device is going to have to be capable of stopping particles as small as the virus to do any good. If you use a filter capable of stopping them at a 30% rate, then doubling up could possibly help a little, though nowhere close to double. But using filters not capable of stopping more than say 5%, with those 5 being stopped because the virus happened to be embedded in a water drop large enough to be stopped, doubling, tripling, quadrupling, etc., is not going to give you any significant protection as a PPE. At best it may do a better job of limiting how much is spread by someone infected wearing it.
Part of the reason N95 masks work as well as they do is because they use electret filters, which attract the smaller virus particles due to the strong electrostatic force exhibited by the fibers used in the making. The masks are constructed in such a way that, if worn properly, the easiest path for air is through the filter.
Once you start trying to use non-electret filters, you need to find other ways to prevent those virus-sized particles from getting through to your nose or mouth. Now you're talking about physical filters so tightly woven or compacted that multiple layers make it harder for enough air to get through to let you breath. Keep in mind, your mask will need to seal on your face well enough also. So, the more difficult you make it for air to pass through the filter, the more likely it will find another path to get in via incoming air...like the sides, by your nose, etc. You need a filter that lets air in easily enough, but also captures a high enough percentage of the virus particles to be useful.
That said, there is a place in SoCal making a mask they claim is stopping as much as 82% of particles according to the only tester they could get...a tester that measures particles from .02 to 1 micron. They claim this virus comes in at around .125 micron. Now I'm not sure how specific their test is as to how much is filtered by particle size in that .02 to 1 micron range (for instance do they get to 82% overall by mostly stopping particles above .125 through 1 micron?), but from what I've seen, this seems to be the most promising DIY mask I've found on the www. And because they aren't selling them, rather offering free instructions, patterns and parts lists on their website for any who want to make them, they aren't driven by profit, thus are far less likely to be offering a bogus solution. Also, they are making no claim they can prevent Covid-19 infection. Just sharing their experience on what they have found seems to work best from likely available materials for DIYers.
Here's a link for the DIY mask instructions:
https://suayla.com/pages/suay-community-mask-coalition
