C Croke and B Kleiner have constructed an example of a CAT(0) group with more than one visual boundary. J Wilson has proven that this same group has uncountably many distinct boundaries. In this article we prove that the knot group of any connected sum of two non-trivial torus knots also has uncountably many distinct CAT(0) boundaries.
"Examples of non-rigid CAT(0) groups from the category of knot groups." Algebr. Geom. Topol. 8 (3) 1667 - 1690, 2008. https://doi.org/10.2140/agt.2008.8.1667