If Wismec would have took the Rx, swapped the 510, and put a bottle in the 3rd battery spot a year ago they would have made a killing. The chip isn't great for tc but plenty good enough at the RX200 price point.
If you want inexpensive dual battery regulated in an RX form factor, Dripbox 160 is pretty darn good. Just beware if you get a painted one and be prepared for edge chipping. I went for the "stainless" and have no complaints. They come on sale in the mid-$30 range. Geekvape S100 might be a good choice too, a little more $ and they're only now starting to ship, but it looks like they gave a lot of attention to the finish and feel. Both have the same kind of bottle system, which works very well IMO.