MUMBAI (BLOOMBERG) - In June, Indian billionaire Mukesh Ambani and his aides ran into an unexpected dilemma when debating where to train the dealmaking lens of his empire next.

Mr Ambani's Reliance Industries was contemplating buying a foreign telecommunications giant when word reached them that Mr Gautam Adani - who had overtaken Mr Ambani as Asia's richest man a few months earlier - was planning to bid in the first big sale of 5G airwaves in India, according to people familiar with the matter.

Mr Ambani's Reliance Jio Infocomm is the top player in India's mobile market, while the Adani Group does not even have a licence to offer wireless telecommunications services. But the very idea that he might be circling ground so core to Mr Ambani's ambitions put the tycoon's camp on high alert, according to the people.

One set of aides advised Mr Ambani to pursue the overseas target and diversify beyond the Indian market, while another counselled conserving funds to fend off any challenge on the home turf, according to people familiar with the discussions.

Mr Ambani, worth US$87 billion (S$120 billion), ultimately never bid for the foreign firm, partly, the people said, because he decided it would be more astute to retain financial firepower in case of a challenge from Mr Adani, who has seen his net worth surge more than anyone else in the world this year - to US$115 billion, based on data from the Bloomberg Billionaires Index.

After peacefully expanding in their respective domains for over two decades, Asia's two richest men are increasingly treading the same ground as Mr Adani, in particular, sets his sights beyond his traditional areas of focus.

This is setting the stage for a clash, with widening implications both beyond India's borders and at home, as the US$3.2 trillion economy embraces the digital era, triggering a race for riches beyond the commodity-led sectors where Mr Ambani and Mr Adani made their first fortunes.

The opportunities emerging - from e-commerce to data streaming and storage - are reminiscent of the United States' 19th-century economic boom, which fuelled the rise of billionaire dynasties like the Carnegies, Vanderbilts and Rockefellers.

The two Indian families are similarly hungry for growth and that means they are inevitably going to run into each other, said Mr Arun Kejriwal, founder of Kejriwal Research and Investment Services.

"Mr Ambanis and Mr Adanis will cooperate, co-exist and compete," he said. "And finally, the fittest will thrive."

In a public statement on July 9, the Adani Group said that it has no intention of entering the consumer mobile space currently dominated by Mr Ambani, and will only use any airwaves purchased at the government auction to create "private network solutions", and for enhancing cyber security at its airports and ports.

Despite such commentary, speculation is rife that he might eventually venture into offering wireless services for consumers.

For decades, Mr Adani's business was focused on sectors like ports, coal mining and shipping, areas that Mr Ambani stayed clear of amid his own heavy investments in oil. But over the past year, that has changed dramatically.

In March, the Adani Group was said to be exploring potential partnerships in Saudi Arabia, including the possibility of buying into its mammoth oil exporter. A few months before that, Reliance - which still gets a majority of its revenue from businesses related to crude oil - scrapped a plan to sell a 20 per cent stake in its energy unit to Aramco.

The two billionaires also have significant overlap in green energy, with each pledging to invest more than US$70 billion in a space that is heavily tied to the priorities of Indian Prime Minister Narendra Modi's government. Meanwhile, Mr Adani has begun signalling deep ambitions in digital services, sports, retail, petrochemicals and the media. Mr Ambani's Reliance either already dominates these sectors or has big plans for them.

In telecommunications, if Mr Adani does start to target consumers in a big way, history suggests that prices could plunge during the early phase of competition but rise again if the two companies secure a duopoly, with India's wireless space currently dominated by three private players.

On the surface, the two men appear quite different. Mr Ambani, 65, inherited Reliance from his father, while Mr Adani, 60, is a self-made businessman. But they also have some remarkable similarities. Largely media-shy, both men have a history of being fiercely competitive, disrupting most sectors they set foot in and then dominating them. Both have excellent project execution skills, and are extremely detail-oriented and dogged in pursuing business goals, with a track record of delivering on big projects, say analysts and executives who have worked with them.

Both hail from the western province of Gujarat, Mr Modi's home state. They have also both dovetailed their business strategies closely with the Prime Minister's national priorities.

Not all Mr Adani's dealmaking overlaps with Reliance, and he has raced ahead with outlays on mergers and acquisitions even as Mr Ambani has stayed cautious on spending heavily overseas amid the uncertain global outlook. The Adani Group acquired the Haifa port in Israel in July for US$1.2 billion. In May, Mr Adani bought Holcim's Indian cement units for US$10.5 billion.

For now, most of Mr Adani's new forays are so nascent that the full impact is hard to immediately gauge. Yet analysts are in agreement that the two men are likely to play a big role in reshaping the Indian business landscape, potentially leaving increasingly vast portions of the economy in the hands of two families.