I took this makro from the warrior's section at wowwiki (
link)
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Switch depends on right click or left click of mouse. Between brackets enter the name of the weapons you want to switch around.
#show [noequipped:shield] <second weapon here(1 hander)>; <first weapon here (2 hander)>
/equip [button:1] <first weapon(2 hander)>
/equip [button:2] <second weapon(1 hander)>
/equip [button:2] <shield or other offhand>