Yes, testing both card idividually will ensure that any problems you might encounter with Xfire will not be because of faulty cards, but because of something else. This is very valuable information.
As for hardware issues.. well, just make sure you have a QUALITY brand PSU (corsair, antec, OCZ, cougar, seasonic, pc power & cooling, ect.) of at least 750w, which will be enough for the xfire on the 6970's. You can probably get away with 700w, but if you don't have a good psu, then you will experience problems running both cards at once.
Also make sure that you have a good CPU that you can possibly overclock a bit. This will keep the CPU from bottlenecking the graphical horsepower that two cards in xfire will bring. Any modern quad core cpu running around 3.6ghz will be fine. This also means you might want to invest into a different CPU cooler if you are running with the stock one.
Thirdly, two cards in Xfire will generate ALOT of heat, especially if there isn't a space between the cards. Make sure that you have a very well ventilated case so that you don't go and overheat yourself. You can use something like MSI Afterburner to measure temps.
I run SLI 560ti's and I can tell you that the top card idles around 37C while the bottom one idles at 27C, and this is WITH a space in between the cards, so yeah.. might want to take that into consideration when buying a new MOBO.
I know it seems like alot to deal with, and even with all these things, more can go wrong with drivers and such, but if you can get it working, its definately worth it.