Well, I bought one 9Kg tub of Atacama Atabites to mass load my Nexus 6 stands and without realising it and before I knew it, I filled the centre columns of both (with a tiny bit left over). So it really depends how much you need to load them. In the end, after experimenting by taking some out again (not easy by the way!) I decided to leave them both completely filled as I preferred the sound, but then I'm using them with some B&W 685s.
I'd buy one tub and experiment how much you want in your front speakers. Once you know how much you want in them, I'd imagine it'll be the same for the rears. If it's exactly half, one tub is all you'll need unless you're going to fill the rear (albeit much thinner) columns as well.