Basically, the DX12 specification allows, in some specific applications, for Multiple Display Adapters to dynamically split the rendering workload and 'work together' to improve performance.
However, this is highly dependent on game developers implementing the feature, and implementing it correctly.
As it stands now, and for the forseable future, DX12 Multi-Adapter should not be considered a viable option to increase your gaming performance.
Aside from DX12 Multi-Adapter support, you would not be able to SLI a GTX 980Ti and GTX 1070 together; only one or the other could be used for gaming.
Therefore, I recommend that you stay with your current GPU for now as the true flagship devices are scheduled to be released early next year from...