You can use multiple joann's coupons if they are from different sources....one from their mailing flyer, one from the newspaper, that kind of thing. But you can't use both from the newspaper on the same transaction/visit or both from a mailing on the same transaction/visit.
Last year Joanns changed their policy for coupons and when youcould use them - used to be that even on sale items they would revert the price back to the original and give you the coupon discount. I'm assuming they were losing a lot of "browsing" buying that way since everyone knew they could go straight to their item of choice and get it for 50% (as it was then) off.
Now, they will only allow coupons on non-sale items, which is honestly the way that the vast majority of other retailers do it. Now, not only do they usually sell items at a smaller discount (even though at least once a month they rotate through their 30 - 40% discount on quilting notions/cutters/etc) but they get buyers to browse the store looking for an item to purchase with their coupon!
It's a good deal whichever way you look at it, in my book!