You wouldn't meaningfully because the people who aren't travelling won't start travelling just because there's more convivial standing space.
To be fair, I don't think large numbers of standing passengers are acceptable on a long distance service. It's acceptable between Wolves and Cov, but not throughout, and XC has far too much standing throughout.
This is one odd case where compulsory reservations would force the issue - want to double the fares income? Double the train length.
But if we are saying XC are so highly loaded that a single 5 car is carrying the number of passengers that would fit in a 10 (which I don't think is largely the case, because there isn't that much standing space on a Voyager - the vestibules are small and the aisle is narrow) then they need to do
more than double capacity to pick up the suppressed demand. But I suspect an uncomfortably overcrowded Voyager is probably operating at maybe 130-150% of the seating capacity, so if you doubled its length you'd get everyone seated and about 50% of spare seats to sell.
One thing that might be worth doing with regard to the subject line is changing the First Class to 2+2 but window aligned and with extra legroom in the airline seats. That could allow it to be only part of the coach and fit in more Standard seats. Another would be replacing the seats with the ones on the Pendolino refurb (or even just Fainsa Sophias) so you can squash in a couple more rows - the original seats are VERY space inefficient. But then all that costs, and inexplicably the DfT won't fund the fix despite them funding it for other TOCs like TPE which, when it only had 185s, was in the exact same position. TPE demonstrated exactly what was needed - fleet doubled = less overcrowding and picking up some suppressed demand.