Koralle can fruit alone, but it still benefits from a partner. Lingonberry is self-fertile but fruit set, berry size, and ripening speed all improve significantly with a cross-pollinator nearby. Red Pearl is the most documented and effective pollinizer. It flowers in bloom group spring-and-fall, which is spring and fall bloom, which is worth knowing if you add a second tree later. Usually grown in USDA zones 2-8.
No single variety stands out as the obvious partner, but that is not a problem here: Erntedank, Linnaea, Sussi, Ida, and Red Pearl all bloom close enough to work. Pick whichever your nursery actually carries. Interplant pollinizer varieties at one plant per 5 to 10 of the main variety. Standard mixed-bed planting works well since lingonberry plants are small.
One thing to keep in mind. Lingonberry yields meaningfully more and larger fruit with a designated pollinizer nearby. Interplant Red Pearl at roughly 1 plant per 5 to 10 plants of other varieties for the best results.
To put that into practice: at a minimum, one variety will fruit, but yields and berry size are noticeably lower than with a companion. Better still, a main fruiting variety (Koralle or Linnaea) with Red Pearl as a pollinizer at a 1:5 to 1:10 ratio. A solo lingonberry bed still produces fruit. Adding Red Pearl is the most cost-effective improvement for yield and berry size.