Beautician Elle Edwards died in hospital after being shot in the head at The Lighthouse in Wallasey Village.
Police say a 30-year-old man was arrested in Tranmere on suspicion of murder and attempted murder.
A 19-year-old woman from Rock Ferry has been arrested on suspicion of conspiracy to murder, police added.
Both suspects have been taken into custody and will be questioned by detectives.
Four other people who were at the pub were also injured - one of those, a 28-year-old man from Beechwood in Wirral, continues to receive medical treatment, police said.
Merseyside Police believe Ms Edwards was not the target of the attack, and Detective Superintendent Sue Coombs described her death as a "cold-blooded shooting".
"Elle's family are still having to come to terms with her tragic loss, and our specially trained family liaison officers are supporting them," she said on Monday evening.
"We will not rest until we bring those responsible to justice."