Can I legally use a second copy of AutoCAD on my other PC?

Yes, if you are the owner of this second PC (e.g. a laptop/notebook) and if you comply with the license agreement of your Autodesk CAD application. E.g. the license agreement to AutoCAD 2010 says in the paragraph 3.1.2:

Additional copies. .... for each Permitted Number of copies of the Software, You may Install and Access, solely within Your Territory, one (1) additional copy of the Software on a second Computer provided that:

1. Such second Computer is owned or leased by You (and not by another person, such as Your employee, if any)
2. Such second Computer is either a portable Computer or a nonserver Computer away from Your usual work location;
3. The additional copy of Software is Accessed solely for the purpose of enabling You (or Your employee, if any) to perform work while away from Your usual work location;
4. The original and additional copy of the Software are used only by the same person, and only one (1) of the Software copies is Accessed at any one time; and
5. Both copies of the Software are Installed and Accessed exclusively with the copy protection device (if any) supplied with the Software.
   

Corporate Subscription users can also use the "Home Use Policy" (where applicable), through which you can use the second local copy of the given CAD application even on a private computer owned by the employee.

Other methods of copying a single software license on multiple computers, or bypassing copy protection, lending software, hacking, cracking, using keygen generators, etc. will break copyrights and violates the law in most countries - you may be subject to criminal penalties.