SLI requires a specific profile for each game, you have to work with Nvidia to create an SLI profile for your project based on the engine and what features you use. That would be difficult in your situation since you’ll likely be making many different projects that you aren’t selling like game are usually sold.
With SLI, both graphics cards have to load the same stuff, so the memory doesn’t stack. With DirectX 12 it has a new feature that can combine any DX12 graphics card where the memory stacks–however, it’s a similar case as SLI, the game has to be configured specifically how it sends processes to each card so it’s not automatic.
A Titan X will be very good for your situation, gaming cards are faster than the workstation cards.