Look in a row, column or box if there are x number of cells containing x possible numbers, where all the cells has the same possible numbers. Meaning, if a cell has only 2 and 3 as possible numbers, look for another cell in the same row, column or box which also has only 2 and 3 as possible numbers. If this exists, these cells must contain the number 2 or 3 as solution and we can remove 2 and 3 from the row, column or box.
1 
9 
2 


7 
8 
6 


6 
7 


9 
4 
1 
2 
8 

4 



7 

9 


9 

5 

2 




6 
7 

3 
9 

1 

1 


9 

6 
4 

9 





1 
2 
8 

4 
1 



5 
9 
6 
6 
2 
8 
9 
1 
5 
3 
7 
4 
Here you see that R3 has two cells which have 3 and 5 as possible solutions. Therefore we can remove 3 and 5 from the other cells in R3, in this case 3 and 5 from R3C4 and 3 from R3C5.
1 
9 
2 
345 
34 
7 
8 
6 
35 
35 
6 
7 
358 
38 
9 
4 
1 
2 
8 
35 
4 
12356 
236 
126 
7 
35 
9 
347 
378 
9 
1468 
5 
1468 
2 
38 
37 
245 
58 
6 
7 
248 
3 
9 
58 
1 
2357 
1 
35 
28 
9 
28 
6 
4 
357 
9 
357 
35 
346 
3467 
46 
1 
2 
8 
37 
4 
1 
238 
2378 
28 
5 
9 
6 
6 
2 
8 
9 
1 
5 
3 
7 
4 
This doesn't work only for 2 numbers in 2 cells. You can do it for the same numbers in 3 cells, 4 cells etc. This sudoku can be solved using DS and SiSo.