Bournemouth, who were relegated from the Premier League last season, won 1-0 over Brentford as they bid to re-join the top flight alongside Norwich City and Watford who also both went down last term.
Swansea City claimed a 1-0 away win at Barnsley to earn a significant advantage ahead of their second leg in Wales.
The winners of the two ties will meet in the Championship playoff final at Wembley on May 29.
Arnaut Danjuma scored the only goal for the Cherries in their home win over Brentford, who narrowly missed promotion last season.
Bournemouth boss Jonathan Woodgate said: “It’s half-time, it’s a slight advantage.
“We know how good Brentford are at home, we know how good the players are and the team is, they’ve been in this situation before and they’ve got that experience. It’s far from over.”
Swansea’s away win came courtesy of Ghanaian star Andre Ayew’s 39th minute winner, as the Welsh club also look to build on last season’s playoffs appearance.
Swans manager Steve Cooper said: “We’ve been here before. It will be good to have a conversation with the players in the week, talk openly and ask them what their views are.
“Not just the ones who were here last year but others who’ve done it (won promotion via the play-offs), like Conor Hourihane who’s done it with his previous clubs, including Barnsley.
“The difference is we were away last year (in the second leg) and this year we’re home, supporters are going to be in the stadium, so that comes into the mix as well.”