That's not a bad idea and thus why I think Infinity asked if we could get new conduits run out of switchboard? Correct Infinity?From the theory peanut gallery, it would also seem that doing the cut-over in such a limited time window would also require getting as much of the work done as possible during 'normal time'. Horsegoer mentions running conduit and having wire in place, and based on synchro's question mentioned the switchboard 10' away.
So what does the job look like if you start a completely new run from that switchboard, going to the second splice location. New conductors already cut to length and ready for termination, or possibly already terminated on your splicing connectors. Pull tape already in the new conduit ready to go. Splice box already made up with openings cut, possibly some sort of box that can take up tolerances should your holes be slightly off. And a spare box on site so that if you screw up the openings you aren't out of luck and time.