How can you keep up with who's who and, more precisely, who's where on NBC's popular Saturday-night sitcoms on Nov. 9? Dinah Manoff of Empty Nest will appear on a special one-hour Golden Girls, Girls' Estelle Getty will visit Empty Nest, and Park Overall of Nest and Betty White of Girls will appear on Nurses. Got it? On NBC's ''seamless night,'' Hurricane Gil and Hurricane Heidi will hit Florida and wreak havoc on the characters of the three sitcoms, all set in Miami. While shows have had overlapping plots before (ABC's Family Matters and Step by Step this season), this is the first time a network has sewn three shows together. ''Warren Littlefield [NBC Entertainment president] asked us to try to run one story line through all three episodes,'' says Tony Thomas, a co-executive producer of all three shows, ''and we figured it might be fun.'' Thomas is pleased with the results, but it was ''an absolute headache.'' No major injuries to any favorite characters can be reported, but, says Thomas, ''Empty Nest bore the brunt of the wetness.''