Here's a simple, two-step plan to double your capital in the next half-decade. First, invest in high-quality, high-growth stocks. Second, wait. Hold on to shares of these companies throughout this period, even amid market downturns.

The hardest part of this strategy might be the first, picking excellent companies. With hundreds of choices in the stock market, it's not always easy to know where to look. That said, here are two excellent candidates to consider: Tandem Diabetes Care (TNDM 3.47%) and Shopify (SHOP 3.82%)

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1. Tandem Diabetes Care

Medical-device maker Tandem Diabetes Care is aiming to make a big difference in its namesake market. The company's crown jewel is its t:slim X2, an innovative pump for the management of insulin-dependent diabetes. The t:slim X2 has seen continued growth in use because of its attractive qualities.

For one thing, it's the smallest pump on the market, making it easier to conceal than those from competitors. In addition, its software can be updated remotely, which allows consumers to add new features as they become available. The t:slim X2 is the only insulin pump on the market with remote software-update capabilities. 

Here's one more perk of this nifty device: It can be paired up with a continuous glucose monitoring (CGM) system -- such as DexCom's G6 -- to predict glucose levels in advance and deliver the amount of insulin needed automatically. This feature is linked to better health outcomes for diabetes patients.

The t:slim X2 helps make the lives of diabetics easier, and that's why Tandem continues to record strong financial results. During the third quarter ended Sept. 30, the company recorded sales of $179.6 million from the t:slim X2 and accessories, representing a 45% year-over-year increase. And the company's net income of $5.8 million during the quarter was a big improvement over the $9.4 million net loss it recorded in the third quarter of last year.

Patient and doctor looking at a tablet.

Image source: Getty Images.

No doubt, more and more people are using these pumps. The company shipped 31,558 of them worldwide during the quarter, 43% higher than the year-ago period. And it ended the quarter with almost 300,000 installed bases of its crown jewel, having added more than 100,000 new customers in the trailing 12-month period.

Tandem has set a goal of reaching 500,000 installed bases by the end of 2024, and it looks well on its way to reaching that milestone. The U.S. market for insulin pumps suitable for type 1 diabetes patients (for whom insulin is a must) remains underpenetrated with roughly 65% of this patient population still relying on painful, multiple daily injections.

The t:slim X2 also appears to be doing well against the competition. According to the company, roughly half of its customers switched from another pump, which suggests that the t:slim X2 is very popular. Meanwhile, the market outside of the U.S. is even more untapped. What's more, the number of diabetes patients will continue to increase in the coming years.

That's why Tandem Diabetes Care should continue to win consumers over -- and with this long runway for growth, the healthcare stock looks well-positioned to crush the market in the next five years.

2. Shopify

Tech giant Shopify has been a top market performer in recent years. Here are several reasons why the company has been on such a roll.

First, Shopify operates in the e-commerce industry, a space that's been growing rapidly as traditional brick-and-mortar stores struggle. Second, the company has an excellent business model. It targets merchants looking to create online storefronts -- practically a necessity for business owners in today's digital world. 

In other words, there's no shortage of potential customers for Shopify, and the company makes their lives easier by providing all the tools merchants need to build a successful online storefront: payment and marketing solutions, shipping and fulfillment services, and much more.

Third, Shopify's platform benefits from high switching costs. Merchants will be hesitant to switch after having taken the time to build an online storefront using the company's tools. 

The combination of these factors has helped Shopify deliver dazzling top-line growth. In the third quarter, the company's revenue totaled $1.1 billion, 46% higher than the year-ago period. This was powered, in part, by the increasing number of customers seeking the company's services.

In the coming years, Shopify should be able to replicate its success by continuing to execute the same business model. And there stands to be plenty of demand. In the U.S., e-commerce sales still only accounted for 12.5% of total sales in the second quarter.

Moreover, the sector is estimated to grow at a compound annual rate of 14.7% through 2027. In that time, I fully expect Shopify's share price to double -- or more.