NEW YORK (TheStreet) -- After almost a year of uncertainty, Dollar Tree's (DLTR - Get Report) acquisition of rival Family Dollar (FDO) is finally slated to close on Monday, but the biggest challenges for the pair may still lie ahead.

In order to secure approval from the Federal Trade Commission, which came last week, Dollar Tree and Family Dollar agreed to sell more than 300 Family Dollar stores across 35 states to New York private equity firm Sycamore Partners within 150 days of the acquisition's close. Now the companies will potentially face an even bigger test, according to analysts.

Integrating Dollar Tree and Family Tree successfully will be the first hurdle, said FBR Capital Markets analyst Dutch Fox in a phone conversation.

"Bearish people are buying Dollar General (DG - Get Report) stock because they believe this is a going to be a sloppy integration. But I do believe that the next three or four data points out of the Dollar Tree-Family Dollar team are going to be fairly positive," Fox said. "Dollar Tree has a great management team that hopefully can fix Family Dollar quickly." 

In a recent investor report, Deutsche Bank analyst Paul Trussell wrote that "upside risks include greater and/or faster than expected synergy achievement; downside risks include challenges in executing the integration and a slowdown in low-end consumer spending."

The big deal came about after Dollar Tree and Dollar General battled it out for the favor of Family Dollar shareholders. After months of back and forth, shareholders finally selected Dollar Tree, whose cash and stock offer valued at $8.5 billion was lower than that of Dollar General's, but whose offer was seen as more likely to get FTC approval. 

Since the deal was approved by shareholders in January, shares of Dollar Tree are up almost 19% and shares of Family Dollar are up 5.4%. Dollar General shares are also up almost 18% in that same time period.

Dollar Tree's acquisition of Family Dollar will create a company almost equal to Dollar General in terms of sales and reach. Together, Dollar Tree and Family Dollar had $4.8 billion in net sales in their most recent quarter, with Dollar General coming in at $4.9 billion in net sales. The combined Dollar Tree and Family Dollar company will operate more than 13,000 stores nationwide, not including the 330 they will sell off. By comparison, Dollar General operates 12,000 stores in 43 states.

Fox said after the first year executives with Dollar Tree will have to do more than simple, cosmetic fixes and dig into fixing some of Family Dollar's core problems, such as its supply chain and quality control issues. "Longer term I do think in time they can come together, but it's a question of how long it takes to get there," he said. "I do think it's going to be a learning process."

Fox said that while Dollar Tree works on improving Family Dollar's business model, competitor Dollar General might stagnate even though its business is generally sound. "Dollar General has a great business model, but the flip side of that is that there are no catalysts in that name," he said.

But the competition between the three dollar discounters is not nearly as important in the long term as how the three companies will compete and adjust to mega-retailers Wal-Mart (WMT - Get Report) and Amazon (AMZN - Get Report), according to Fox. He said the growth of online and on-demand shopping will hurt the three companies if they do not adapt, particularly Family Dollar.

"Dollar General is in a lot more super-rural areas, and Family Dollar is more urban, suburban," he said. "So the greater risks in terms of the long-term horizon is toward Family Dollar, because their stores are in more population-dense places, which is where if you're Amazon you want to go."