Namun has the same meaning with "tapi (but)". Walaupun has the same meaning with "meskipun (although)" Well, in your sentence "Ali malas. namun, ahmad rajin" is already correct. But don't put punctuation mark (.) and (,) in the sentence cuz u already have word "namun" there. Namun function as conjuction which connects two sentences to be one sentence.