"Dallas" never acknowledged any plot lines from "Knots Landing" and the only two "Knots Landing" stars to ever appear were Ted Shackelford and Joan Van Ark ("Gary and Val"), although J.R. did mention Karen Fairgate's name when Gary came to visit him after J.R. was shot.
The three most important "Dallas" episodes to watch before watching "Knots" are probably "Reunion" (Parts 1 & 2), which was the two-part second season opener and which introduced Gary and Val for the first time, although Gary was initially played by David Ackroyd, and "Return Engagement" from season 3, which was the episode that spun Gary and Val off into their own series.
Gary came back to Dallas when J.R. was shot, Gary and Val attended Lucy's wedding, Gary came back when Jock was in South America and divided up Ewing Oil, Gary returned for the reading of Jock's will, Gary came back for Bobby's funeral (during the "dream" season) and both Gary and Val showed up in the series finale, "Conundrum", when J.R. had an "It's a Wonderful Life" trip. As for Val, other than the above mentioned episodes, she appeared by herself early in a third season episode, "Secrets", to make up with Lucy, but was usually seen as an accessory to Gary in episodes he appeared in.
On "Knots Landing", the "Dallas" crossovers didn't affect storylines because early in the series run, "Knots Landing" was not a continuing drama/soap opera, but instead was a one hour drama with "bottle episodes" (that is self contained storylines that were resolved at the end of the hour and didn't cross over from week to week). After the fourth season, crossovers from "Dallas" stars stopped, although they did refer to and incorporate Bobby's death (from the "dream" season on "Dallas") into the storylines of "Knots Landing"'s seventh season, when Gary grieved for his "late" brother and Val renamed her infant son, Bobby, in memory of the "late" Bobby Ewing.
There were only nine "Knots Landing" episodes which featured "Dallas" stars. Three were from season 1, including the Pilot (in which Patrick Duffy had a cameo at the beginning), "Community Spirit", in which Larry Hagman came to Knots Landing to drill for oil on the beach and "Home Is for the Healing", the only episode where Charlene Tilton comes to Knots Landing and is reunited with her parents. Season 2 included four more "Dallas" crossovers, including "Kristin", in which Mary Crosby guest-starred weeks after she gained international fame by shooting J.R., "A Family Matter", in which Larry Hagman visited Knots Landing and dealt with Gary and Abby, "The Loudest Word", in which Patrick Duffy guest-starred to lend support to Gary when Val was hospitalized and "Designs", in which Larry Hagman came back to Knots Landing to steal Sid's invention and have an affair with Abby. There were no "Dallas" crossovers during season 3, but in season 4 there were two more: "Daniel", in which Larry Hagman came back to Knots Landing to confront Val after buying the company that published her expose on the Ewings, and "New Beginnings", in which both Patrick Duffy and Larry Hagman appeared. That episode aired immediately after the "Dallas" season 6 episode "Jock's Will" and the only time there was a crossover storyline between the two series, which dealt with Jock Ewing's will.