When starting a game, for cards in my hand, the effects are applied in order of acquisition. So, if I have something that costs 9, it shows, with the animation 9 -> 4 then it tries to apply the Scepter of Summoning and cannot find anything costing more than 5, so it does nothing.
For the cards in the deck though, because they don't go through the animation, the order is "correctly" applied: any minion that costs more than 5 costs 5 (static). Then cards get their cost divided by 2, from their static cost (dynamic).
As a result, except for the cards in the starting hand, no minion can cost more than 2. And no non-minion card can cost more than 5.
For cards from the starting hand, no card can cost more than 5.
I suspect the order of acquisition of these 2 treasures will change how the starting hand behaves, though.
I found a few bugs but this one clearly was obvious (when it's in front of you ^^).
I had: Robes of Gaudiness and later I got Scepter of Summoning.
When starting a game, for cards in my hand, the effects are applied in order of acquisition. So, if I have something that costs 9, it shows, with the animation 9 -> 4 then it tries to apply the Scepter of Summoning and cannot find anything costing more than 5, so it does nothing.
For the cards in the deck though, because they don't go through the animation, the order is "correctly" applied: any minion that costs more than 5 costs 5 (static). Then cards get their cost divided by 2, from their static cost (dynamic).
As a result, except for the cards in the starting hand, no minion can cost more than 2. And no non-minion card can cost more than 5.
For cards from the starting hand, no card can cost more than 5.
I suspect the order of acquisition of these 2 treasures will change how the starting hand behaves, though.
tl;dr: Robes of Gaudiness and Scepter of Summoning do work together. Except for starting hand.
Like you said the order probably matters and that behavior sounds consistent with how other effects work in the game. So not a bug.