What your after is an engineering solution.
It depends on what tooling you have around. Id make up the tooling
as it would then give me another product to add to my stock. If it was
me, Id make the 2 tubes from sheet strip, one in sterling and one in
gold, as an interference fit one inside the other, then using a ring
roller with one wheel convex and the other concave roll the ring
between them to get the domed profile. They wont come apart.
If you dont have this type of tooling then you could make up a
hinged pair of dies out of steel. polish it well and use your bench
vice to apply the pressure to move the metal.