Kelvin was a shipping lawyer and then a movie critic for The Straits Times, while Leon has bachelor's degrees in economics and biology and a master's in marine biology.
So when they decided to make movies together, Kelvin quips of their parents' reaction: "It's the reverse of striking the first and second prizes in the lottery. Not only does the elder son decide to give up lawyering to go into film, but he suddenly drags in his younger brother as well."
They have a younger sister Sherry, 36, who works in logistics.
After completing his first feature, the youth drama Eating Air (1999), Kelvin decided that making films was his true calling. And a corporate entity was necessary to deal with administrative details, from taxation to intellectual property rights.
Describing Leon as the one who is good with money, Kelvin saw in his mind a classic director-producer set-up. And Leon, who had caught the tail-end of that debut movie's production and found it "quite interesting", was excited by the idea.
Multi-Story Complex, founded in 1998 by Kelvin with his Eating Air co-director Jasmine Ng, was renamed Boku Films in 2003, with Leon and producer-director Kat Goh coming on board and Ng exiting. "Boku" means "I" in Japanese.
The triangular core turned out to be a stable set-up.
Kelvin says: "It would be more difficult if Boku Films was just the Tong brothers. We're lucky to have a third person - she is the de facto referee. When we discuss things, usually Leon and I have very strong opinions, so whichever way she drifts is the way Boku is going to drift. It sort of works."
Working with your sibling also means there is complete trust. As Kelvin puts it: "Film has a lot of very grey bits about it, especially when it comes to finances, and my brother's really good at keeping our company afloat. That takes a lot of weight off me as a creative."
While he declines to give figures, Leon says: "We are profitable enough to still be making films since 1998." Their output includes horror flick The Maid (2005, $2.2 million at the local box office), horror comedy Men In White ($500,000), cop horror flick Rule #1 ($2008, $1.1 million), thriller Kidnapper ($440,000) and nostalgia drama It's A Great Great World ($2.5 million).
Next up is a horror flick with Hollywood funding, The Faith Of Anna Waters, starring acclaimed series Mad Men alumna Elizabeth Rice. It is slated for a release here on May 12.
Apart from their mutually beneficial working relationship, Kelvin and Leon say it is also nice for their parents to always see them together because the opposite tends to be true. "Siblings do drift apart because they have their own families and everything. It's inevitable and quite common," says Kelvin, who is married with a daughter. Leon is married with no children.
As for the downside, Kelvin jokes: "I can't fire him and he can't fire me."
The director-producer set-up that he envisioned so many years ago has worked well for them, but there might be changes afoot. Kelvin notes: "I sense there's a film-maker in Leon as well. He's not just the number-cruncher or the guy who claims the GST. When we brainstorm, Leon has very good ideas.
"For so long, Boku Films has been about Kelvin Tong. The big conversation we should be having soon is, should we be looking at other kinds of films? I think this is the first time he's hearing this."
In response, Leon, who is the quieter of the two at this interview, says decisively: "I don't think it's my forte, I've never been interested in that role." He is happy solving problems as a producer and helping to realise Kelvin's cinematic visions.
In describing their shared journey as film-makers, Kelvin tells an anecdote from when he was about 10. Leon had decided that they would sleep in the garden instead of the house and they had built a shelter from discarded cardboard boxes and stuck it out for two weeks.
Kelvin says with a smile: "It was like a camping trip and, so far, Boku has been like a camping trip as well. I don't know how amused our parents are and I've wondered, 'Why are we sleeping in a cardboard box?'
"But at the same time, it's very fun because it's with my brother."
•The Faith Of Anna Waters will be released on May 12.
Always finding common ground