If you're talking about a GA-78LMT-USB 3, with one CPU fan and one chassis fan header, you'll need a splitter like the one linked below and plug both case fans into the chassis fan header. It's a three pin splitter, but your case fans are probably three pin also, and you can plug them into the four pin header - it'll just leave one pin exposed but you don't need it.
If you have a different mobo, please state the exact model number.
http://www.amazon.com/Splitter-Extension-Sleeving-Connectors-Length/dp/B00C46DX4S/ref=sr_1_cc_2?s=aps&ie=UTF8&qid=1420503782&sr=1-2-catcorr&keywords=fan+splitter+3+pin