I think if children were brought up learning more than 1 instrument we'd see more people who mastered more than one instrument. Imagine if Horowitz could also play the violin as Heiftz
I fear that this very thing was indeed imagined - at least by those who believed that a certain story circulated some years ago was actually true (though, much as I'd like it to have been, I suspect that it was merely apocryphal). Late in both the lives of the above, Heifetz and Horowitz were (supposedly) approached by RCA to record the complete sonatas for violin and piano by Beethoven. Horowitz was apparently delighted at the prospect. Heifetz, however, put the kiss of death on it by saying that he felt that Horowitz's violin playing was not up to such a project.
That said, legend (and maybe in this instance not so fatuous a legend as the above might have been) has it that, in the days after Heifetz had decided to hang up his public bow, he played the piano parts from some of Beethoven's violin sonatas from memory in his masterclasses.
Rostropovich was indeed more than a merely capable pianist (I write "was" only because I am not aware that he still plays that instrument publicly). Enescu wasn't just a violinist. There may well be some other interesting examples of the kind of performer about which this thread asks, although I am inclined nevertheless to think that "few and far between" probably remains the order of the day here...
Best,
Alistair