If you need OR, you'll need to use a prereq, rather than an exprreq.
Code:
validif (#skillranks[Craft(chemical)] >= 6)
validif (#skillranks[Craft(electrical)] >= 6)
(Replace the names with the proper Ids, of course)
That way, if either test is good, the validif will set the prereq to valid and exit the script, which gets you "OR" behavior.