Carl Icahn has taken a 10 percent stake in energy company Dynegy, becoming the second big institution to acquire a large share. Hedge fund manager Seneca Capital previously took a 9.3 percent stake.

Both investors are taking aim at Blackstone's deal announced in August, to buy the company for $4.7 billion, or $4.50-a-share. That amounted to a 62 percent premium over its closing price the previous day, Reuters notes. Both investors think the deal undervalues the company. And both will likely raise other issues.

Dynegy's "go-shop" period reportedly ended on September 22, with no interest expressed by others. "Conditions have deteriorated since Blackstone's offer, citing low and declining commodity prices, continued economic weakness and a challenging financial position," the company said in a recent letter to shareholders. And yet the stock has been trading higher. The market may be expecting another bidder to emerge.

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