Will using 2 or more synth controllers (like an NI Kontrol S61 & S25) together with Sonar allow you to use all the knobs and sliders on both, or will only one work?

Important: controllers should be different. There was many reports Windows / Sonar do not understand when there are 2 controllers of the same kind. Depending from Control Surface plug-ins you use for both, some workarounds should be applied/kept in mind, other then that 2,3 or mode can work in parallel.
 

I can second that. 2 devices of the same make is asking for trouble - unless the device driver was specifically written for multi device use ... even then it may not be guaranteed that it works without hiccups (e.g. i'm using 2 ESI MIDI interfaces with multi device driver but on win10 often only 1 is recognized after start up requiring unplugging or power cycling the hub they are plugged into)

Devices of different make should work fine and do work fine here

Good to know. Thanks guys, tho you'd think 2 of the same brand should be possible.

soens
Good to know. Thanks guys, tho you'd think 2 of the same brand should be possible.

And probably will work, as long as the brand support that (so that should be clarified in each particular case). From Sonar point of view, the only problem are identical names for corresponding MIDI ports.

I wonder if it makes a difference if the controllers use multi-client MIDI drivers supplied by the manufacturer, vs. relying on class-compliant USB MIDI drivers?