Rob Grant and Doug Naylor created the TV show Red Dwarf. They also wrote two Red Dwarf novels together but then parted ways.
The first two novels were called Infinity Welcomes Careful Drivers and Better Than Life. After their split, they each wrote a third installment which was a direct sequel to from the second book. Doug Naylor wrote The Last Human, and Rob Grant wrote Backwards.
I suppose you could call them altquels as they are both equally valid continuations of the story (I have my views about which is better, but that's not relevant). I remember being quite confused when reading The Last Human after Backwards, before realising they aren't meant to be consistent with each other.
Backwards ends inconclusively, when Lister and the Cat escape from soon-to-crash Red Dwarf, leaving behind Rimmer and Kryten who have already been killed by a computer virus (it's hard to explain if you've never seen it). Lister and the Cat travel to another dimension and discover an alternate, living Rimmer and Kryten in a universe where Lister and the Cat are long dead. Rimmer tells them that something is about to happen, but Kryten interrupts him and says he will explain it all later. The book then ends.
I have bolded this part below. Did Rob Grant ever explain how the story was going to move forward?
'This is the mining ship Red Dwarf. You have encroached on our airspace without warning, which we must consider an act of aggression. Ergo, we surrender. Totally and unequivocally. Do you copy?’
Lister grinned and switched to 'send'. 'Rimmer, you are such a worldclass smeghead.’
‘Lister? Switching to visual.’
Rimmer's image appeared on screen. He peered forward, baffled and confused. 'Lister??’
Kryten squeezed in beside him. 'Sir? Is it you?’
Rimmer said. 'How can we be sure it's you? Tell us something only you could know.’
Lister thought. 'I know gazpacho soup is served cold,' he tried. Rimmer gritted his teeth and nodded violently. 'It's him all right, the obnoxious little gimboid.’
Kryten looked perplexed. 'I don't understand, sir. You're dead.’
‘We buried you some years ago. You and the Cat. You were both trapped in a lethally addictive game. There was nothing we could do to save you.’
The Cat poked his head over Lister's shoulder. 'Who are you calling dead, dog-chew head?’
‘You're both alive? But how?’
‘We'll tell you when we get on board. If we don't get out of this ship soon, the Cat's conkers are going to be crushed beyond recognition.’
‘Well.' Rimmer's forehead wrinkled. 'You picked a rare old time to show up. We're about to be...’
Kryten cut him off. 'There'll be time aplenty for that, sir.' He leaned towards the screen. 'Head for docking bay four seven five, sir. I'll have a vindaloo sauce sandwich waiting for you. Signing off.’