The cash is burning a hole in your pocket hey
If you are talking about using 2 wireless devices simultaneously in backrack 4, there is no problem in doing that.
First of all make sure you check the hardware compatibility list before choosing a USB wireless dongle.
Also a good idea to do a few google searches on the specific device you are looking at (in a show I saw the other day,
someone said "I binged it" just doesnt sound right...unless Chandler were to say it of course)