Try disabling SLI for Fallout Tactics.
To do this right-click your desktop and click NVIDIA Control Panel. Select Advanced options if asked to. Now click "Manage 3D settings" and select the "Program Settings" tab. Once there, click Add and locate the file bos.exe in the Fallout Tactics directory and double-click it. Now, in the list below, find "Set Multi-GPU configuration" and set it to "Single-GPU". Click Apply. Tactics should now work properly.