It should be noted that SLI is only relevant for real-time viewport performance. If you wish to enable SLI for 2/3/4 NVIDIA graphics adapters this does not interfere with Thea Render. However SLI no benefit for the renderer itself and furthermore you will only limit yourself as you may not be able to add another graphics adapter from a different card generation. Furthermore in more recent generations of NVIDIA Pascal GPUs, enabling SLI for more than two cards has proven to be a challenge that has no additional performance benefits.