Pretty sure a 66 and 92 cannot work like that [assuming you mean the 92 leads 'dead' and the 66 stays tucked in behind but provides the power] ...otherwise they wouldn't need to have the 66 leading on the Dollands Moor-Wembley section then have the hassle of switching locos around at Wembley.
If and when they do use the S&C, I suspect it'll either be 66 hauled throughout, or the 92 would come off somewhere in the Crewe/Warrington/Preston area (e.g. Lostock Hall) and then pick up the return empties working a couple of days later.
Would be a lot of hassle if they switched at Wembley (as done already), then switched around again at (say) Warrington, then back again at Carlisle to the 92 (if going the Beattock route).
Makes more sense when the Dumfries route is open again, as the 92 can just come off and the 66 can complete the rest solo up the S&C and then on from Carlisle as it does anyway when going this way.
I'm pretty sure the 92 is the only loco permitted to haul the china clays solo on the 'hilly' WCML sections. And even then...