Finally we were able to finish the plumbing in the galley. It was a challenge to find all the pieces required to fit our double sink (one sink bowl is smaller than the other). We ended up buying a complete double siphon set (symmetrical), discard the unnecessary piecies and ordering a smaller strainer waste for the smaller sink bowl from the same manufacturer (McAlpine).
The screws that fixes the wastes to the sinks also had to be replaced, as the included screws were too short. We ordered one screw from a plumbing webshop for one of the bowls and improvised an M6 screw with a washer and a nut for the other bowl.
Now we have everything connected with a 38mm drain and 20mm bilge pump hose, all above the water line and secured with two hose clamps.