Each of the bids, which include stock and cash, is now valued at roughly $26.5 billion.
The bidding began in November, when drugstore operator CVS offered $21.2 billion in stock for Caremark. Express Scripts joined the fray in December.
The two companies are vying to join forces with Caremark, the second-largest US pharmacy benefits manager, to increase their bargaining power with drug makers and edge out competitors like Wal-Mart Stores Inc. and Medco Health Solutions Inc., the largest benefits manager.
To help sweeten its offer, CVS is adding $1.50 to its earlier special cash dividend offer of $6 a share payable to Caremark shareholders following closing of the merger. That adds $643 million to its earlier bid, which included stock.
CVS also said if the deal is closed, the combined company will make a cash tender offer of $35 a share for 150 million, or about 10 percent, of its outstanding shares.
Express Scripts, another pharmacy benefits manager, on Wednesday said it would boost its offer of 0.426 shares of its own stock and $29.25 in cash by 0.481 cents a day, starting April 1, until the deal is closed.
CVS said its latest proposal constitutes a "best and final" offer.
CVS also said it has set a shareholder vote on the deal for March 15. Caremark shareholders are to meet March 16.
Woonsocket, R.I.-based CVS is the largest US pharmacy chain.
Shares of Caremark rose 86 cents, or 1.4 percent, to close at $62.16, then dropped 12 cents in after-hours trading. CVS rose $1.09, or 3.5 percent, to $32.41 and rose another 9 cents after hours. Express Scripts shares rose $1.23, or 1.7 percent, to $76; they rose 2 cents after hours.