Warren Buffett isn’t necessarily known for his growth investing. But he has identified several stocks that, for decades at a time, turned out to be growth machines. There are two stocks in Berkshire Hathaway‘s portfolio right now that every investor should consider. They’ve been monster growth stocks in the past. And if you use the correct investment strategy, they can still produce immense long-term gains for your portfolio.

These two stocks have produced monster growth for Buffett

Back in 2011, Buffett bet a small amount of Berkshire’s portfolio on two competing businesses: Visa (V -0.52%) and Mastercard (MA -0.62%). Even back then, he likely had no idea how successful these investments would become. Initially, Berkshire purchased shares of Visa at about $40 per share, while its Mastercard position was initiated at roughly the same amount. Since 2011, the combined basket of Visa and Mastercard has risen in value by about 2,000%. That’s 20 times Berkshire’s original investment.

It’s not hard to understand why these businesses proved so successful. The payments industry in general is very consolidated. According to data compiled by Statista, Visa and Mastercard control more than 86% of the U.S. credit and debit card market. Just two competitors control the remaining 14%. It has been this way for decades. In fact, Visa and Mastercard now combine for a higher market share than they did in 2007, when they controlled roughly 81% of the market.

Dominant market shares make sense in an industry that requires scale to operate. Payment networks function best when they can connect to as many endpoints as possible. Therefore, the largest networks tend to win, something business experts call the network effect. Network effects are why Visa and Mastercard control most of the market, and why they’ve done so for decades. Merchants and consumers want their payment methods to work no matter what, and the largest networks make sure that this occurs.

Combining scale with an asset-light business model — both companies rely mostly on software versus physical infrastructure — has resulted in huge profit margins and returns on equity. Both companies, for instance, have consistently generated profit margins between 45% and 55%, with similar returns on equity while using little debt for leverage.

In total, Visa and Mastercard have created a natural market duopoly with incredible economics that should continue long into the future. But if you want to invest in either stock, there’s a key investment strategy you should employ.

How to profitably invest in Visa and Mastercard today

Visa and Mastercard have great business models with durable competitive advantages. But there’s a reason Buffett chose to invest in both, and not one over the other. Earlier this year, Visa was hit by a federal lawsuit charging it with anticompetitive practices. Both companies have faced litigation pressures in the past, none of which derailed their duopoly positions. But it’s important to note that the best thing to bet on isn’t a particular company, but the industry in general. Betting on both allows your portfolio to win in aggregate, whether Visa or Mastercard in particular performs better than the other.

Right now, I would take this investment strategy even further, taking a small position in an alternative payment network like Block (SQ -3.05%) in combination with a larger position in both Visa and Mastercard. Over time, if Visa and Mastercard finally cede market share, there’s a good chance they lose this market share to a company like Block, which not only operates Square — a major payment network used by merchants all over the world — but also the peer-to-peer payment platform Cash App, which helps everyday consumers pay each other directly or buy Bitcoin and other alternative currencies.

Don’t get me wrong: I still love Visa and Mastercard long-term. But the best strategy is likely to build a small basket, similar to what Buffett did in 2011. A basket dominated by Visa and Mastercard shares, plus a small exposure to competing alternative networks like Block, should do well for decades, if not the next century.