I play Riven onto a battlefield. With two different items, one is Svellsong and the other Cull. Can I target two different units with Riven's ability?

Yes — you can target two different enemy units.

Svellsongur copies Riven's printed "When I attack" text onto itself, so when Riven attacks you get two separate triggers: Riven's own ability and the copy on Svellsongur. Each is an independent triggered ability with its own target chosen when it goes on the Chain, so you can pick a different enemy unit for each — no rule forces them to match. Cull doesn't add a trigger, but it does count toward the damage: both triggers deal 2 per Equipment attached to Riven (Svellsongur + Cull = 2), so each of your two targets takes 4. (Same principle as Svellsongur + Ezreal, Dashing, per FAQ 8854.)