Peter Lynch was a top-performing mutual fund manager. In addition to growing wealth for his investors, he has shared a wealth of knowledge with the general public through his best-selling books. Among his nuggets of wisdom is that "the best stock to buy is the one you already own." In other words, it's often better to add to an existing position than a new stock in your portfolio. 

With that wisdom in mind, now could be a great time to add to your highest-conviction investments. Two of my highest-conviction ideas are American Tower (AMT -0.55%) and Blackstone (BX -2.58%). Because of that, I think now would be a wise time for fellow investors to add $500 or so to their positions (or consider adding these stocks to their portfolios if they don't already hold them).

Trading at near a historically high yield

Shares of American Tower have slumped 30% from their 52-week high. That has the infrastructure REIT trading at a cheaper price and higher dividend yield. The dividend yield is currently around 3%, putting it near the highest level since the company initiated a payout upon converting to a REIT a decade ago. 

The cell tower and data center operator has done an excellent job increasing its payout over the years. American Tower grew its dividend by 12.5% last year and expects to deliver another 10% increase in 2023. Powering that growth is the growing demand for data infrastructure as we use more data for streaming, cloud computing, AI, and more. That's enabling American Tower to add more tenants to its existing infrastructure and invest in developing additional capacity. 

The company expects to invest about $1.7 billion this year in building additional infrastructure. That includes constructing about 4,000 international cell tower sites and modestly increasing its U.S. data center capacity. Those investments should help grow its cash flow in the future, enabling American Tower to keep increasing its dividend. 

The long-term trend remains in tact

Blackstone stock has fallen almost 40% over the past year. The primary culprit has been a surge in redemption requests by investors in its non-traded REIT, BREIT. That investment vehicle has been a victim of its own success. It has performed extremely well over the past year, leading its shares to hold their value. That's causing some investors to seek to cash in so they can redeploy the proceeds into potentially higher-returning opportunities. 

While the redemptions could slow Blackstone's growth in the near term, the company should continue to benefit from the expansion of alternative investments. Institutional investors like pension funds have been steadily increasing their allocation to alternatives over the years because of their higher returns and lower volatility compared to the public stock and bond markets. That trend should continue in the future. Meanwhile, high-net-worth investors are starting to follow institutions into alternatives, especially as more options like BREIT become available. 

For example, research from Morgan Stanley estimates that the private investment market will grow at a 12% annual rate over the next five years. Meanwhile, it foresees high-net-worth investors more than doubling their allocation to alternatives over the next five years to 8%-10% of their portfolios. 

In the meantime, Blackstone trades at a relatively attractive valuation. It generated $5.17 per share of distributable earnings last year. With the stock around $80, Blackstone trades at about 15.6 times its earnings. That's historically low for an asset manager. They tend to trade at more than 20 times earnings. Because of that low valuation, Blackstone currently offers a dividend yield of 5.4% based on its dividend payout over the last 12 months.   

A great time to add

American Tower and Blackstone are benefiting from strong long-term growth tailwinds. While they've experienced macroeconomic, market, and interest rate headwinds in the past year, those issues should eventually disappear. Because of that, both appear poised to continue growing their earnings and dividends at attractive rates over the long term. That makes them excellent stocks to add to right now.