The problem with the HVL-20 is that it only fires the HVL-58 off camera if the 58 can 'see' the 20 (they are optical wireless systems). I also don't like that I'm using 2 AAA batteries in my HVL-20 and having to waste that power just to fire the HVL-58. That also means I have to carry spare AAA batteries so that the HVL-20 can keep up with the HVL-58.
If you just need the flash off to the side a bit, get one of the Yongnuo remote flash cables off eBay or even pay a few bucks for a non directional wireless attachment.
I have the remote cable and its fine. Of course you lose coupled distance indexing but can still shoot TTL.
I also bought an 8 battery External Flashlight Battery Pack SF-X.
That little beastie means I have high speed flash for motor driven shots (recycle time reduced to about half) or, for big events, means I have 12 batteries on board instead of the std 4. It does come with a std tripod screw option to attach to the base of the camera but with a vertical grip attached that's not practical. The long curly cord allows it to be hung from a belt or put in my pocket and a belt mount soft case is part of the package.