Does thwonk trigger radiant dawn twice

Yes — as long as the two stuns land on two different attacking units. Thwonk!'s Repeat lets the spell execute its effect a second time, and those executions resolve as separate, sequential game actions (game actions are never simultaneous, Rule 303.2), so Radiant Dawn's "when you stun one or more enemy units" condition is met twice, generating two buffs.

The catch is the target: a unit that's already stunned can't be stunned again, so if there's only one attacking unit, the second execution does nothing and you only get one trigger and one buff. With two (or more) attacking units you stun each on a separate execution and get two buffs. Note each buff still lands on a friendly unit of your choice, and no unit can hold more than one buff.