Against (Perkins Observatory):
http://perkins.owu.edu/solar_viewing_safety.htm
Be careful that you use the right kind of glass! Welder's glass is numbered from 1 to 14 with 14 being the darkest. It is only number 14 glass that is dark enough for solar viewing! And NO STACKING! A pair of number 7's or a 10 and a 4 together DO NOT have the same protection as a single piece of number 14 (see unsafe methods for more details).
Favorable (Royal Astronomical Society of Canada):
https://www.rasc.ca/tov/safety
If SN14 filter is not available, it is possible to combine lower shade numbers to get roughly the same level of eye protection from solar radiation, e.g. combining SN 6 and SN 8 filters. However the image quality may be considerably poorer than that seen through the single SN14 filter
I could not find a table or something explaining which kind of protection gives each number; according to the Canadian website, the only concern is about how much infrared light goes through, ultraviolet does not seem to be a problem in almost any case (I was surprised to read that).
Note: It's not my intention to open here the discussion on what could be other safe methods to watch the eclipse, this is well explained everywhere around. I read too late about the eclipse to order specific glasses.
Edit: One of the answers here report the following formula: (more insight at this link)
13 or darker is safe enough. Also, you CAN add up welding glass, using the formula S(sum) = S1 + S2 -1. S(sum) should be greater than or equal to 13
Stacking is fine. Just don't expect the numbers to be simply additive, that's all. E.g., indeed a 10 and a 4 together are not the same as a single 14. What you really need is bring the brightness down to the level of a TV screen showing a white image, that's all. Don't stack too many glasses - if it doesn't work with two of them, don't stack more than that. – Florin Andrei Mar 19 '15 at 20:10