I am not sure if Wurm Online has native support for SLI or not. However, I discovered that the following settings in NVIDIA Profile Inspector seem to work.
I verified with GPU-Z that the load is being spread across both of my NVIDIA GTX 970s as they both have around 50% GPU Load. What you need to do is create a profile named "Wurm Online" and find your j2plauncher.exe file from Java since this is what the game uses and add it to the new profile. Once you apply these settings, you can restart the Wurm client and verify with GPU-Z that the load is being spread across both video cards. It works for me!
Keep in mind, you need to select the number of GPUs to use in SLI rendering mode. I only have two (one GPU per card) but some cards have dual GPUs, etc.