Koper confirmed the permanent signing of 22-year-old goalkeeper Piana from Udinese before following up with a loan deal for the 19-year-old Pejičić, a Slovenian attacking midfielder who has made a single appearance for the Friuli club's first team.
The two moves continue a close working relationship between the clubs, with Udinese sending young players to Koper to gain first-team experience in Slovenia's top flight.
For Pejičić, the move represents a second consecutive season away from Udinese in search of regular football, having spent last season on loan at Maribor.
Despite his limited senior involvement in Italy, the playmaker is regarded as one of the more promising talents to come through Udinese's academy pipeline in recent years, and the club will hope a productive campaign in Slovenia can accelerate his development in the way similar loans have done for other prospects.
The connection between the two clubs has strengthened in recent transfer windows, with Udinese using Koper as a proving ground for fringe players who are not yet ready to feature regularly in Serie A.
Piana's permanent departure removes him from Udinese's goalkeeping pathway entirely, while Pejičić retains the option to return to Friuli once his loan spell concludes at the end of the season, depending on his progress and the club's assessment of his readiness for senior Italian football.
Koper announced the Pejičić loan with an Instagram post welcoming the midfielder to the Slovenian PrvaLiga club, continuing a pattern of transfer activity between the two sides that has developed over recent seasons as Udinese looks to place development players in competitive first-team environments abroad.
Pejičić's move follows a similar path to several other Udinese loanees who have used spells in less high-profile leagues to build match sharpness before potentially returning to challenge for a place in the Friuli club's senior squad, with the length of his single senior Udinese appearance underlining just how early he remains in that development process compared with more established members of the first-team squad.