Are they both grammatical and mean the exact same thing?
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If you mean "risky and speculative", then you should say 危険で投機的な, because that's one of dedicated meaning 連用形 has.
Saying 「危険な、投機的な事業」 (putting a comma is a good practice) for this meaning is not prohibited, but it either sounds like adding words one by one while you're speaking, which isn't very nice for written language; or could mean "risky or speculative".
Otherwise, 危険な投機的な事業 might well be interpreted as "risky speculative business", that is, 危険な modifies all what comes after.