The combination will spread the cost of funding what could be Poland's largest-ever deal, according to Thomson Reuters data, and narrows the field of bidders to five from seven going in to the second round.
Telenor's operational expertise and Bain's dealmaking experience were also factors in the combination, a person familiar with the matter said.
Poland would represent new exposure in a large emerging market for Telenor, which faces stagnating sales in the Nordic region.
Flattening sales at home have already sent most of Western Europe's large telecom companies into Africa, Asia and Latin America in search of growth.
Telenor already generates over half its revenues from the emerging markets, including a hefty stake in Vimpelcom, soon to be the world's fifth largest mobile company when its own deal with Wind Telecom has closed.
For Bain, combining with Telenor means a greater chance of success at a time when strategic bidders are returning to auctions in greater numbers and when financial leverage is less than what it was during the buyout boom.
Cable group Liberty Global snatched Kabel Baden-Wuerttemberg away from CVC last month, while Diageo paid $2.1 billion to take Turkish liquor producer Mey Icki off TPG's hands, pre-empting a sale to another buyout firm.
"I don't think we have ever really paid more than a corporate buyer who really wanted to own something," Joseph Baratta, senior managing director at Blackstone, told the Reuters Global M&A Summit recently.
Blackstone placed a first round bid for Polkomtel with TPG, but the pair has since exited the process. KKR dropped out this week.
Binding bids for Polkomtel are due on May 6 the Polish press has previoulsy quoted the company as saying.
Telenor and Bain may face competition from Polish media tycoon Zygmunt Solorz-Zak, TeliaSonera and private equity firms Apax and Providence.
All of these parties were invited through to the second round, other people have told Reuters previously. Apax is expected to bid on its own, with backing from its investors.
Mobile phone giant Vodafone is one of the selling shareholders in Polkomtel.
An exit from the company would add to the $23 billion dollars that Vodafone has already raises from sales of minority stakes in Japan, France and China.
Polish state-controlled firms KGHM PKN Orlen PGE and Weglokoks are also selling out of Polkomtel.
Poland's Treasury Minister Aleksander Grad told journalists on Thursday it would be a "good idea" for Polkomtel's state-linked owners to hand back most of the proceeds to their shareholders in the form of special dividends.
Poland has been relying on privatisation income and dividends from state-controlled companies to help keep down its borrowing needs.
All the parties either declined to comment or could not be reached for comment.