The two parties have urged other regional outfits to join their alliance, sources in them said.
"We have constituted the UDP-HSPDP coordination committee with legislators from both parties as members to chalk out on the strategies for the poll," UDP president Donkupar Roy told reporters here.
"We will meet soon and finalise the modalities and also prepare the spadework for the alliance," he said.
Roy, who is also elected the chairman of the coordination committee, said that HSPDP chief Ardent Basaiawmoit will be the co-chairman of the committee.
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"At the central executive of HSPDP we found that the alliance would be for the welfare of the people of the state and we as the regional parties have to take it up together," Basaiawmoit said.
Such an alliance is not new for the regional parties in the state and UDP and HSPDP will draw from the past mistakes and work according to the common minimum program of the alliance, Roy said.
The other 14 seats in the Garo Hills region are understood to have been left to the understanding between the UDP and the National Peoples Party led by Lok Sabha MP Conrad K Sangma.
"We are closely associated with the NPP. Although seat adjustments will be difficult we are also looking for some poll understanding in the Garo Hills region," Roy said.
Asked about the BJP-led North East Democratic Alliance (NEDA) of which UDP is a part, he said that NEDA was not a platform for discussing on pre-poll alliances but for discussing issues of common interests for the region.