I'm using Malwarebytes, but hear there is free scanner from BitDefender in App Shop
Any opinions? Would it be safer to have two scanners to use from time to time?
Is BitDefender safe or could it do something bad with my Mac? It's even in App Store so, respectable enough?
As noted, BitDefender is a well know anti-virus/malware company, so it is perfectly fine to use their products.
The one thing that I will note is that programs bought through the App Store can be limited due to Apple restrictions (in particular the requirement that they be "sandboxed"). For many programs, this is not an issue. It can, however, be an issue for more "lower level" utilities (which can include anti-virus/malware programs). The BitDefender program that you linked to does address this issue under the "Constraints" section of the product description, which I will quote here:
Due to sandboxing in the App Store, the app has access only to the locations that the user has explicitly granted access to. Grant access to the entire disk for increased protection;
Thus, I would generally tend to recommend getting Mac anti-virus programs outside of the Mac App Store (i.e. buy/download directly from the anti-virus company's website).
As to running multiple anti-virus/malware programs, it is OK if they are only "on demand" programs (i.e. not constantly running in the background, but rather scan manually initiated by the user). Just like for Windows, there is a good chance that running two real-time (i.e. running in the background constantly scanning) anti-virus/malware programs will have conflicts and create programs. Of course, many free anti-virus/malware programs are "on demand" only.