Is it okay to use two lock washers?

It causes problems. Lock washers increase the friction between the nut or bolt head and the surface. If you stack two of them you are creating a slippery surface between the lock washers instead.

Using Lock Washers

I had a problem like this too. This lady and her boyfriend decided they worked hard enough in life and were going to take a 4 month vacation on my dime. That's how long it took to get them out once they stopped paying. But since I was going to re-rent the house back out I figured I would use this time to put on a new roof and have that old nasty driveway broken up and redone. Fortunately I have friends and family in both the roofing and concrete trades. So I was able to have my cousin and his crew do a complete tear-off and re shingle the roof between the hours of 8 a.m and 10 a.m every morning for a week and a half. And this was a job that could of been done in a day had they worked all day on it. Then I had 45 feet of driveway, a new walkway, and a patio put in. Since my buddies were doing it on the side they only had time to do so on the weekends and we always started early. Now this was all done toward the end of summer and the house has no A/C. So I took great pleasure out of hearing the windows slam being called a this and that. I figured why disturb my new tenant that hopefully will be paying the rent (the new one did and still does for 2 yrs now). As much as I wanted to I never did use any self help eviction methods, and you shouldn't either. A judge can rule that because of your actions they were not liable for rent during that period and order you to repair such disabled items and give them another chance. Thats the last thing you want to happen so just let them be and screen your tenants more thoroughly next time. Good Luck!

Just a few points on using lock washers.

First your not supposed to re-use lock washer.

Next when you do use lock washers, all ways use a torque wrench to tighten the bolt.

If your worried about a bolt coming loose, its better to use Loctite thread locker.

No not really. It's not necessary, but it doesn't hurt either.