Nvidia has not yet disclosed the detailed specification of those new hardware blocks added for raytracing. Furthermore the programming interface for these RTX units is, as far as I know, fairly specific to raytracing of typical 3D models. So it is currently unknown if the RTX cores will ever be useful for fractal purposes.

However, the early benchmark data suggests that Nvidia's 2000 series cards are notably better at compute tasks than the 1000 series. The performance improvement for compute seems to be quite a bit larger than the performance improvement for games.

However, the 2000 series cards are also very expensive. Maybe one can get three or more midrange GPUs for the price of a single 2080Ti, and maybe those three have a higher combined compute throughput...?

I posted some price performance comparisons on the Indigo Renderer blog recently: https://www.indigorenderer.com/node/2201

The RT units in the new RTX won't be useful for fractal rendering, but overall it's still a monster for GPU compute and worth the investment IMO.

If you have more than one GPU in your computer, you can load other copies of the program and assign them to the other GPU's.  But you have to manually assign the work for each GPU,  i.e. render frames #0 to #x on one GPU, frames #x to frame #y on another, etc.

Netrendering in openCL will probably be implemented  before the  multi gpu option. This all will take some time.

Maybe one can get three or more midrange GPUs for the price of a single 2080Ti, and maybe those three have a higher combined compute throughput...?

This would be my approach. Depends a bit on work methods.  More than one gpu allows you to be rendering a final section of the animation while making changes to another section of the animation.