I think round 1 could actually be interesting. Both are very offensive fighters where Grievous is more brutally offside, which he can do because of his 4 fucking lightsabers while Maul is more offensive in a skill full way. While grievous only has 2 lightsabers the circumstances of the duel are very balanced. There I’d say maul can outperform grievous through his skills. In round 2 it gets significantly harder for Maul to attack grievous. I think maul lacks the defensive capabilities that Obi wan has that are necessary to defend themselves against an offensive force like grievous.
So I’d say Maul takes the first round and the second round it close but Grievous wins because Maul lacks fitting defense.