Octane always needs to load the entire scene on each GPU, so SLI is not an useful option.
About GTX 780ti, yes, if you add another one of them to you system, you are roughly doubling your rendering speed reaching 200 points in OctaneBench.https://render.otoy.com/octanebench/res ... ingleGPU=1
Wile adding two 980ti you shoud reach a score of 250, but you also double the available VRAM from 3GB to 6GB.
If 3GB is enough for you, you can stay with 780ti for now, waiting to update with more powerful GPUs later on.
When you add more than one GPU to your system, be careful about the form factor of the card, classic layout like Titan gpus is the way to go, even better hybrid or hydro cooling. In short, avoid the cards with custom design and more than one fan. If your cards got overheating, you loose rendering speed.