I know contextually this phrase is in the past tense from when Star Trek is set, but how come the present tense isn't accepted?
It is -- "Kahless and Lukara defend each other" (in the present simple tense) is accepted.
what is the difference between saying lukara and kahless and saying kahless and lukara?
Keep the order of nouns or sentences joined by a conjunction (e.g. "and", "or", "but") the same when translating -- luqara' qeylIS je is "Lukara and Kahless" and qeylIS luqara' je is "Kahless and Lukara".