If those TV promos featuring Garry Shandling and Brian Benben grudgingly watching the premier episodes of each other's HBO series seem awfully convincing, there's a good reason. The Dream On cast and crew are doing a slow burn over the attention the cable network has lavished on Shandling's The Larry Sanders Show. ''They have been promoting (Sanders) a little more than us,'' says Dream On executive producer Kevin Bright. ''I understand it's a business decision, but it stinks.'' Asked about the fuss, Bridget Potter, HBO's chief of original programming, said, ''If there's a sibling problem, then HBO's the parent, and I don't think we want to discuss our family problems in public.'' The Larry Sanders folks had no comment. Potter insists the problems have been put behind them and invites viewers to see the spots as on-air therapy: ''In a family, when there's an issue, the best way to deal with it is directly.'' The animosity seems to end at the studio door: The stars are friendly neighbors in the upscale community of Sherman Oaks, Calif.