A squirrel will begin mating with other squirrels at 10 to 12 months of age.
Some squirrels may only mate with one mate through their entire life but most squirrels including male and female squirrels mate with several different squirrels and have more than one mate in their lifetime.
So if you see a squirrel mating with one squirrel it's likely that sometime later that same squirrel will be mating with another squirrel once they find one.