Sosa, capped 27 times by Croatia, remains Köln's first choice to provide competition for new signing Gideon Mensah, who started at left-back in the club's 3-2 opening-day win over Hoffenheim. The Croatian spent five seasons at Stuttgart, where he built a reputation as one of the Bundesliga's most productive crossers from the left flank, before joining Crystal Palace last summer.
His first season in England proved difficult, and he has yet to feature for Palace this term, leaving the door open for a return to Germany.
Sporting director Thomas Kessler is working to complete the Sosa deal, but Köln have identified de Wit as their contingency should those talks break down before Tuesday's window closes. A Köln delegation travelled to the Netherlands on Monday to advance discussions, according to Kicker, with the AZ full-back only likely to become an active target if the Sosa move falls through entirely.
De Wit, who came through Ajax's academy before spells in Portugal and Spain, has spent the past four seasons at AZ Alkmaar, where he is contracted until 2029.
Predominantly a left-back capable of playing further forward, he registered six goals and 13 assists in 52 competitive appearances for the Dutch side last season, output that reflects a game built around getting forward as much as defending — originally trained as an attacking wide player before settling into his current role.
The pursuit of a second left-sided option comes just after Köln completed the signing of Mikey Moore on loan from Tottenham, part of a busy deadline day for the newly-promoted club as they look to add further competition across several positions before Tuesday's cut-off.
Köln opened their Bundesliga campaign with that 3-2 win over Hoffenheim, a result that has only sharpened the club's focus on reinforcing the left side of defence before the window shuts.
Whomever of Sosa or de Wit Köln end up signing, the aim is the same: to give their head coach options as they look to sustain the sort of positive attacking play down the left flank that Mensah has provided so far this season.