Rumour has it that the original writing team were aiming for Indiana Jones -- a film series that was proving staggeringly successful at the time.
There was just one last question before the film could be made, the same question faced by all films with a British hero -- how to shoehorn an American character into the plot.
I've tried to summarising the plot several times now but the pain is too much if you want to know the details then check the wiki page. You can see the appeal it allows them to drop an American 'ordinary Joe' (from the modern day no less) into the Biggles story. There we can contrast his lovable modern American irreverence and 'can do' attitude with the stiffest of stiff upper lips of the Great War. And to be honest with a bit of effort you could make that work, unfortunately Biggles: Adventures with Gibbons doesn't.
What makes the bloody awful 'time twins' plot so bloody, bloody awful is that it shows no understanding of how stories work. Unlike it's direct inspiration 'Back to the Future' the time travel aspect of Biggles has no bearing on the plot whatsoever it's merely a macguffin to get our Ordinary Joe back to the Western Front, the time jumps are arbitrary and unrelated to the dastardly German's secret weapon plot which has the result of sucking all the tension out of both plots.
Now I know, from watching the film, that the actual screen writers were not total incompetents and therefore the problem lies with a gibbon -- probably a producer. When you consider the amount of money, effort and talent that goes into even a modestly budgeted film it becomes clear why gibbons are a menace and must be driven out of the industry before it is too late.
Gibbons: just say no.