Is it possible to run a successful portfolio with just two shares? I wouldn’t recommend it, but if I decided to try, National Grid (LSE: NG) and GlaxoSmithKline (LSE: GSK) would probably be on my shortlist of stocks to consider.

Both companies appear to offer a combination of reliable long-term growth and generous dividend yields. Both firms are also very large. Together, National Grid and Glaxo account for 5.7% of the FTSE 100’s total market capitalisation. This makes them less likely than smaller companies to fail, although long periods of underperformance are still possible.

Not likely to underperform?

Indeed, many investors believe that GlaxoSmithKline has been underperforming over the last few years. Although the dividend has been maintained, the firm’s shares are currently trading 15% below their July 2013 high of 1,745p.

However, things now seem likely to change. Glaxo completed a major asset swap deal with Swiss firm Novartis last year. The firm’s sales rose by 4% and while profits were lower, a number of new products made a strong contribution for the first time.

I believe Glaxo’s restructuring has positioned it to deliver a sustained period of growth. Adjusted earnings per share are expected to rise to 85.3p this year, covering the firm’s forecast dividend of 81.9p per share for the first time in three years.

There’s also one more big change in the pipeline. The group’s chief executive, Sir Andrew Witty, is expected to leave next year. If GlaxoSmithKline shares continue to underperform the market, I suspect its next chief executive will come under pressured to break up the group.

Fund managers such as Neil Woodford hold the view that Glaxo would be worth more as two or three companies than it is as one. I suspect he’s right, although I like the defensive nature of Glaxo’s diverse portfolio.

In the meantime, I believe GlaxoSmithKline’s 5.5% dividend yield and long-term growth potential make the shares a strong buy.

Heading in the opposite direction?

While GlaxoSmithKline may have underperformed the market, National Grid hasn’t.

Over the last five years, shares in National Grid have risen by 69%, compared to just 3% for the FTSE 100. The two other large listed power utilities in the UK, SSE and Centrica, have also lagged National Grid. Centrica has fallen by nearly 30% over the same period, while SSE has gained just 17%.

National Grid also has a couple of other attractive features. The firm’s US utility business helps diversify the group’s operations and reduce its dependency on the UK market. Another attraction is that unlike electricity generators like SSE and Centrica, National Grid’s direct exposure to coal, oil and gas prices is very low.

I can’t see any reason why National Grid’s business can’t continue to perform strongly. My only reservation is that the shares are no longer obviously cheap. Dividend growth has now slowed to just 1% to 2% a year. And National Grid’s 4.3% yield is well below the 6% on offer from SSE.

However, analysts have a positive outlook for the next couple of years. Forecast earnings for 2015/16 have risen by 5.5% over the last year. There’s a real possibility that National Grid will continue to outperform its utility peers.

Indeed, I'm not the only Fool who's bullish about National Grid and GlaxoSmithKline.

Both stocks were recently chosen by the Motley Fool's top investment experts for their unique retirement portfolio, 5 Shares To Retire On.

Indeed, while two stocks may not be enough for a portfolio, I believe that the five companies featured in this report could be enough to provide a market-beating performance.

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Roland Head owns shares of GlaxoSmithKline and SSE. The Motley Fool UK has recommended GlaxoSmithKline. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.