The second effect of this card restricts your opponent to either 1 Spell Card or 1 Trap Card each turn; not one of each. This effect applies during both players’ turns.
The second effect of this card only restricts the activation of new cards. It does not restrict the number of times that the effects of face-up Continuous Spell and Trap Cards can be used.
The second effect of this card only applies to cards activated after “Great Shogun Shien” is face-up on the field. If your opponent activates a Spell or Trap Card, and then later you Special Summon “Great Shogun Shien” (or flip him face-up), your opponent can still activate 1 more Spell/Trap Card this turn.
If “Great Shogun Shien” is face-up, and the activation of your opponent’s Spell/Trap Card is negated (by “Magic Jammer”, etc.), they can still activate 1 more Spell/Trap Card after that.
Please see "The Six Samurai – Irou" for additional rulings.