如何在不打断另一个的情况下同时调用函数X次

假设我有这个函数：

int num;
void aa()
{
  num=0;
  //Wait 1 sec
  num++;
  //Print ("num= "+num ..
  //Wait 1 sec
  num++;
  //Print ("num= "+num ..
}

如果我同时调用aa((两次，我得到：

num=1 from first time called
num=2 from second time called
num=3 from first time called
num=4 from second time called

但我希望结果是

num=1 from first time called
num=1 from second time called
num=2 from first time called
num=2 from second time called

所以我想要一个函数的多个实例，但没有一个中断或与另一个混合。

注意：我使用的是Unity和C#。

void aa()
{
  int num;
  num=0;
  num++;
  num++;
}