Any fully upgraded exotic produces 2 shards when broken down.
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You don't have to fully upgrade to get the extra shard, just buy an exotic armor piece from Xur, use 4 motes on it and dismantle for two shards, it's not that hard!!!
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Gjallarhorn will only give 1
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Weapon. Armor is only a chance at two, may be (often is) only one. Of course it takes so much rep to unlock exotic armor that nobody in their right mind would try farming shards with them when a heavy weapon takes ten motes or less!
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Telemetry and 6 motes to get two shards from truth. Plus once dismantled you'll get 2-4 motes back