Over on rec.arts.sf.written, this YASID has been asked twice, with no answer. Anyone?
Do you know this book? It's a paperback book I had in the mid-60s...about two (eventually) physically similar guys, from two completely different environments, who trade places for a month or two.
The one from our (1950s) world is a meek, sickly British prole, living his life of quiet desperation in a terrace house with his wife. The other is a bored starship captain, weary with ennui at how he can have anything he desires, and all has become so predictable. They both decide to commit suicide by jumping into the river -- but instead of the quick impact they both wind up flying through a featureless gray void, passing each other midway -- and then each is somehow mistaken for the other, futuristic medicine rapidly rendering our hero into a strong, vigorous guy who gets into his new role, using unexpected tactics to win space battles. Meanwhile back in England our other hero beats up the neighborhood bully, winning 'his' wife's and the neighbors' adoring respect . Then somehow, again they're hurtling through the void, passing (and this time recognizing) each other, and upon arrival finding that each has improved the other's lot, happy endings all around.
"Tomorrow" may have been a word in the title. Any idea what it was?