This can't be the right way to go about this...

argonautSumTypes.scala

import scalaz._
import std.option._                                                                                                                                                                                                                                                             
import argonaut._,Argonaut._

object JsonDb {

  sealed trait SumType
  case class Option1( foo:String ) extends SumType
  case class Option2( bar:Int    ) extends SumType

  private implicit val option1Codec = casecodec1( Option1.apply , Option1.unapply _ )("foo")
  private implicit val option2Codec = casecodec1( Option2.apply , Option2.unapply _ )("bar")

  implicit def sumTypeCodec  = codec2[Option[Option1],Option[Option2],SumType](
    (fooOpt,barOpt) => (fooOpt,barOpt) match {
      case (Some(o1),_) => o1
      case (_,Some(o2)) => o2
      case _            => throw new RuntimeException(":(")
    },
    (sumType) => sumType match {
      case o1:Option1 => (some(o1),none[Option2])
      case o2:Option2 => (none[Option1],some(o2))
    }
  )( "option1" , "option2" )

  //scala> Option1( "hello world" ).asJson
  //res0: argonaut.Json = {"option1":{"foo":"hello world"},"option2":null}

  //scala> Option2( 1337 ).asJson
  //res1: argonaut.Json = {"option1":null,"option2":{"bar":1337}}

}