I'm a manager and an employee threaten me twice within 2 years? As a manager can I sue the company, since I feel the company is brushing it off?

Not enough info to go on.

You are the manager, what steps have you taken? Has the employee been reprimanded in any way?

Also was this the same employee? What type of threat? Why are they threatening you? And not to sound coy on this, but two threats in 2 years is not really that bad. I would get to a month, sometimes to a week in my work when I was a manager. So much depends on what you are considering a threat and if it is real or merely an employee feeling stress and expressing themselves.

And most important, what do you do when threatened? What powers do you have.

Maybe, but most likely not. You are management are required to try and do things, so when company is "brushing it off" what have you done. Have you issued any written warnings, (are you allowed to) Have you asked company to put them into some anger management program. Is everything you asked for, in writing (for proof)

Also threats, if not immediate and putting you in fear for immediate harm, may not even be illegal. Like I am going to punch you out after work.
But if the threat is real, then why have you not merely called the police when this happened. Working for a company does not take your rights away to call the police if there is a danger present.

The issue is what tort, would you sue for. Even if you were a little scared, you were not to a point it would cause extreme emotional distress.

As to a hostile work environment, you may file a complaint with EEOC (not sue them for money) to see if it can be investigated.

Sure you can sue anyone you want, but in this case, you won't win.
A manager is expected to handle threats - write them down, discipline the employee with written warnings or probation or firing. Policies need to be clearly written by the company. If they don't have any policy, you work on one and present it to them as a suggestion, showing that you are MANAGER material.
If a threat is specific and dangerous, you write down the day and time and words and get as many witnesses as possible, in writing, and fire the employee. If you think that would result in harm to you, you file a police report too.

(EEOC would require about 200 pages of paperwork and would take about 2 years to make a decision.)

I'm a manager and an employee threaten me twice within 2 years? As a manager can I sue the company, since I feel the company is brushing it off?

Are you not happy with your job? Do you really think the company will retain you if you bring them into court, especially for a frivolous suit?

There is no enough information here to make a judgement about whether you have a case or not. But the likelihood of winning such a case is very thin.

So what exactly is the threat? Killing you? Turning you in for cheating on the expense account? Serious enough to warrant police action? You are the manager aren't you? What is company policy regarding a threat and did you follow it?

Not a lot of info. What was the threat? Was it a physical threat, or more along the lines of, "I'm going to key your car", or, "I'm going to put laxatives in your coffee"? What do you consider a threat?

Also, as a manager, why are you not doing something about this? You're a manager! Why are you not writing up this person? Do you not have the power, as a manager to fire employees that are causing issues?

Sounds like you're whining a lot, and expecting someone else to handle the issue for you, which makes you a very poor manager imo.