Answer :
I tried each possible combination and quickly found the answer, that's how it looks:
11 + 1 = 12
11 - 1 = 10
10 + 2 = 12
10 - 2 = 8
9 + 3 = 12
9 - 3 = 6
8 + 4 = 12
8 - 4 = 4
(...)
So the numbers Sven was thinking of are 8 and 4.
11 + 1 = 12
11 - 1 = 10
10 + 2 = 12
10 - 2 = 8
9 + 3 = 12
9 - 3 = 6
8 + 4 = 12
8 - 4 = 4
(...)
So the numbers Sven was thinking of are 8 and 4.