Two developers with competing proposals to build southern Maryland's first regional shopping mall in Charles County joined forces last week. The move came as a third developer, from Prince George's County, increased efforts to lure national retailers to the 200-acre Penney-Ward tract at the Charles County line and build the huge shopping complex there.

Economic observors in Charles County say the southern Maryland area can only support one such regional shopping plaza, so the decision by Pennsylvania developer George Zamias to buy 90 acres from Waldorf's Interstate General Corp. strengthens the hand of the new partners by cutting the number of competitors.

Both James P. Lee, vice president of Melvin Simon & Associates of Indianapolis, the Prince George's competitor, and Zamias agree that they will have to entice at least three major national department stores to their prospective sites to make the shopping mall gamble worth the risk.

Neither group will say which, if any, stores they are close to signing but both say negotiations are going on with chains of the Woodward & Lothrop, Hecht's, Sears, Bambergers or Garfinckel's caliber.

Whichever group pins down the anchor stores first is expected to break ground late this summer in time to open for the 1988 Christmas retail season.

Charles Stewart, vice president of Interstate General Corp., said the agreement with Zamias came because "he realized our site was more ready to go than his. . . . We have the sewer and water on line and the highway turn-offs and the commercial zoning for a 500-acre tract."

Zamias, who passed one zoning hurdle earlier this month when his petition to get an 81-acre parcel off Berry Road zoned commercial was approved by the planning board, withdrew his application after cutting the deal with Interstate Corp.

"We're delighted with the arrangement," Stewart said. "We won't own the main mall but will lease and develop all the out-parcel properties."

Ray Tilghman, executive director of the Charles County Economic Development Commission, said he is happy with the turn of events. "I always wanted a joint venture rather than a fight between two developers. A local fight would make it more likely that Prince George's would get the mall. Now we are in the stronger position."

Damian Zamias, a son and partner in the George D. Zamias Development Co., predicted that the complex would be open by spring 1987 provided his group gets written letters of intent from a major department store in time to begin construction this summer.

Zamias said his group envisions a two-tiered shopping plaza with four or five nationally recognized chains as tenants. Local observors say that Sears Roebuck is likely to be among the stores, although Zamias would not comment on local speculation. Negotiations are also ongoing with The Hecht Co. and its owner, May Department Store Co., a Midwest chain of stores based in St. Louis, in an attempt to get either store to enter the southern Maryland retail market.

Roads remain the big snag with the joint venture in Charles County. The proposed mall would be part of St. Charles City's Westlake Village development, bordering Route 301 and Smallwood Drive.

Stewart said his group has offered the county $1 million to extend Smallwood Drive from Middleton Road west to Route 210 (Indian Head Highway). "That extension has been on the county's master construction plan for three years. . . . It is just a question of lighting a fire under someone."

Stewart said constructing a 2.5-mile link between Middleton Road and Route 301 and then adding another five-mile western extension to Route 210 would give the shopping complex "the best intersection in southern Maryland."

"We truly expect it to be another multi-use complex like Tysons Corner. You have to remember that Tysons was a farm and Crystal City was nothing 20 years ago. . . . High rises are a realistic expectation for Charles in 25 years," Stewart said.

The area has reached a critical population and has a very strong market that could easily support a regional center the size of Tysons Corner, Tilghman said.

Both Tilghman and Stewart say the area is losing customers to Springfield, Fredericksburg and White Flint malls who might just as easily be plunking their dollars down in Charles County.

"It's your basic captive market down here. We've got 250,000 people in southern Prince George's, Calvert, St. Mary's and Charles counties with no place to shop."

Stewart says with the 1980 census, Charles moved up to third place in Maryland -- behind Montgomery and Howard counties -- in per-family income.