Find and store the humanoid root parts of both the characters.
local player1 = [player_path].Character:FindFirstChild("HumanoidRootPart")
local player2 = [player_path].Character:FindFirstChild("HumanoidRootPart")
local distance = (player1-player2).Magnitude
.Magnitude finds the distance between the players in studs.
Thanks to the pivot update, it’s relatively simple to compare distances between models. You’re using this for character models, but this can be used for models in general.
local model1 = workspace.Model1
local model2 = workspace.Model2
local distance = (model2:GetPivot().Position - model1:GetPivot().Position).Magnitude