Off the top of my head I'd say Complex mixer with 4 pages, Curve type of Fixed
pages 1 and 2, mux of "Replace" for SW1;
- position 0, curve fixed to "-100"
- position 1, curve fixed to "+50"
then for pages 3 and 4, use a mux of "Add" for SW2
- position 0, curve fixed to "0"
- position 1, curve fixed to "+50"
result should be
0 0 [(-100) + (0)] = -100
0 1 [(-100) + (+50)] = -50
1 0 [(+50) + (0)] = +50
1 1 [(+50) + (+50)] = +100