these two felines were introduced at 6 weeks old in order to give more enrichment for the lynx and for education purposes, however they got along fantastically from the start and were eventually moved to permanently live with one another.
a much less exciting story, but a much cuter one as well.
The calico I had for 18 years also did a lot of trilling and that odd closed-mouth mrow. The tortie we had would trill and sing in the corners of certain rooms. We figured she liked the echoes.