I think a war hammer fits Sardonyx very well, it's big and flashy just like her personality.

As for Sapphire's weapon, maybe her future vision(and things related to that ability) is what she contributes to the fusion. She uses it often, and it seems she's more aware of when to use it while fused.

Sapphire seems like a passive sort of Gem, looking at Garnet's page, it seems more likely to me that things like future vision, Temporary Power Transfer, and Structural Sensation would be the sort of things she would contribute. With her speed and future vision, I'd wager she wouldn't be a fighting Gem in the physical sense. And looking at her fusion with Sardonyx and Sugilite, I feel she doesn't contribute anything to the gauntlets and that they're more of a "functional stylistic choice" by either her, Ruby, or both.

tl;dr: Sardonyx weapon fits her very well. Sapphire seems like a passive Gem and likely contributes passive abilities. i.e. her "weapon" doesn't affect Ruby's gauntlets.

Ok guys, stop to think a little.

SU is, beyond being a story plot, a show, a very well written show that since the beggining makes foreshadowings and obviously hides stuff.

The reason Garnet has never summoned each R and S weapons separatedly is, obviously, because the writers of the show, despite leaving clues to Garnet being a fusion, also wanted her to be seen as a single gem, hence not giving that away.

Secondly, we dont know and havent seen fusions summon their weapons after the original time so who knows if Opal just takes a bow from her forehead or Sugilite a flail from her chest. Additionally, SU is not just about the plot, it sets to impress. And i bet youve noticed that the first time a fusion is formed its always way more flowery and animation-wise more impressive, while the times after that are very casual. The same can be said about weapons. Although this argument can easily be canceled by the cameo of another fusion gem, it is possible that the first time the audience is presented to a weapon they over do it and do the whole fusion of the weapons. Im pretty sure that in future episodes animators will not want to waste time, budget or patience on drawing these sequences again, but we'll see. Sometimes you have to consider SU is a show thats drawn by people and that some moments are done to impress. Notice how pearl never did her weird dance to summon objects from her gem again.

I think fusions dont have to combine the fusees weapons because for sardonyx and sugilite they did it to impress steven (hey steven wanna see something cool? -sugilite) (And jokes arent the only thing ive got.-sardonyx then does act ) and opal could have done it to show him her weapon And in cry for help during tower of mistakes sardonyx jumps up saying let the show begin with obviously no hammer then we see her jumping on the communication hub poles WITH her hammer so she maybe didnt have to combine garnet and pearls weapons

I think they're saying the flashy show the Gem's fusions put on when they show their weapon is just a one time thing and for Steven's, and our, benifit. So basically if the Gems wanted, they could summon their weapon already combined.

Okay, but I guess they can't, it is depent of what is there weapon, like Pearl and Amethyst weapon are too big to take out at once, and Garnet weapon is just stick on her hands.

I don't know, I feel like it could but maybe not. I guess technically they don't have to pull their weapon from their gem. Jasper's gem is on her nose, but she doesn't pull her helmet from it. She summons it similar to how Garnet summons her glasses. Maybe the Gem decides how the weapon materializes?

So it depends on how the weapon summon, because both Amethyst and Pearl need to pull out the weapon, so Suglite, Opal and Sardonyx need to do that too, but Ruby (and possibly Sapphire) weapon just summon on her hand, so Garnet don't need to pull out her weapon, Jasper too.