You can use pond liner from HD. Cut it into strips about 6" to 8" wide. Staple it to the sidewall extending over the crack and protruding out to the door. With the door open you can staple the other side of the strip to the door, just leave a little bump sticking outward over the crack. That way when you close the door the bump sticks into the closet area.

This will seal the cracks in the door.

The cover the crack between both doors you can use thin wood and cut strips and cover one door and hang it over to cover the crack in-between the doors. Weather strip on the exposed side will seal the last bit. You just need to remember what door to open first.

If you want to keep the tight then add a slide bolt to the fixed door (the one with the wood strip) on the top and bottom.

If you need to you can get a cam lock and install it on the door so nobody can get in with out wrecking your grow closet.

I hope that helps you some.