If you use a single KC62, it probably will not move as much air as a pair of LS60s.
The total surface area of all of all eight woofers in the LS60s is approximately 667 sq. cm.
The total service area of both woofers in a KC62 is approximately 321 sq. cm., which is about 48% of the surface area of the LS60s woofers.
My guess is that the KC62 has longer throw, which will make up for some of this deficiency, but whether the throw is long enough to make up for all of the deficiency I do not know. If not, then you may end up with worse bass response using a single KC62 assuming that you high-pass the LS60s.
If you really have your heart set on a single KC62, you may want to investigate running the LS60s full range in the "Less" Bass extension mode without high-passing them, then use the the KC62 to fill in the lows. That way, you will be getting bass from all eight of the LS60 woofers in addition to the two 6.5" woofers in the KC62.
A potential issue, though, is that the KEF software does not provide much in the way of subwoofer tunability. So, it may or may not work well depending on your room modes and the positioning of the KC62 with respect to the LS60s, which affects the phase relationship between them. KEF does provide a 0 or 180 degree phase switch, but such a switch has never been adequate in systems in which I have tried to use one. You could, though, run an external DSP unit which would give you a greater level of tunability. With external DSP I think you could make it work well.