If your character, such as Dexter Drake, has spells that use only one hand and he has two copies of the same spell, such as Shrivelling, is it possible for him to cast both spells, and get the full benefits of the combat dice if he succeeds on both spells during one round of combat?

Short answer: Yes.

Long answer: Yes, as long as he succeeds in casting both of them and pays the sanity cost, and uses both hands to "maintain" both of the spells. No reason why he can't :-)