So, as the title says, I ordered my binders last week.  Hopefully, they'll be in by the end of the week.  I'm not sure how fast Underworks free shipping is.  Anyway, I've got a big chest (somewhere between DD and DDD).  Should I use two binders, one of top of the other, to get a more flat chest?  Would that be safe?  I don't want to wonk up my ribs.

No, that is very very dangerous. Maybe two sports bras can be worn at the same time (that isn't as dangerous i think), but I don't recommend even that for your own safety. By all means do *NOT* wear more than one binder at a time and please don't wear binders for more than 8 hours per day. Binding can be very dangerous if you aren't cautious. Stay safe and bind safely.

Absolutely not. Using just one binder puts enough stress on the ribs and back to cause lasting damage if you're doing it over a long period of time. There are several guys on this board (myself included) who either have gone, or will be going to a chiropractor post top surgery to straighten out the issues caused by long term binding. Under no circumstances should you wear two at the same time.

Just alternate between the two. It will help them last longer and stay fresher between laundry days.

Thanks so much for your replies, guys!  I was unsure, and had heard about double binding somewhere.

Generally double binding is not a good thing.
BUT! I can say that if you're in the position of having maybe 2 old binders that are stretched out double binding might be okay. Or if you have a binder that doesn't bind well and wear a very tight rashguard or compression shirt under it.

Nope, nope, nope. Don't double bind. It could cause things like broken ribs, shortness of breath and possible fainting, it could constrict the heart causing palpatations, etc. Just try to avoid double binding One binder should work perfectly, if it's the right size. Stay Safe!

-Ty