If a player summons a Star Seraph Scepter while having a Star Seraph Sovereign in his hand, both their effects activate and form a chain according to SEGOC with the owner choosing the order of the chain links. When Scepter resolves adding another Star Seraph monster to the hand, if that Star Seraph is another Sovereign, it can also be Special Summoned via its own effect, starting a new chain after the current one finishes resolving.
This is because the hand is not a "public" zone to the game state. The 2nd Sovereign (the added one), even though was not in the hand when the summons took place will activate its effect on a new chain after the current one finishes resolving exactly because it was in the hand at the end of the chain's resolution. Since the game state cannot "see" the existence of that 2nd added Sovereign to the hand throughout this whole chain, it assumes it was there and allows it to also activate its Trigger effect, triggered by the summons of Scepter/Sovereign.
This is because the hand is not a "public" zone to the game state. The 2nd Sovereign (the added one), even though was not in the hand when the summons took place will activate its effect on a new chain after the current one finishes resolving exactly because it was in the hand at the end of the chain's resolution. Since the game state cannot "see" the existence of that 2nd added Sovereign to the hand throughout this whole chain, it assumes it was there and allows it to also activate its Trigger effect, triggered by the summons of Scepter/Sovereign.