It will depend on the amount of each of the two solutions.
For example:
If 500 mL is of 0.4M solution is combined with 500 mL of 1.2M, the resultant solution will be 0.8M.
Calculate by percentages:
Total new solution = 1,000 mL
500 mL = 50%
50% x 0.4 = 0.2
50% x 1.2 = 0.6
0.6 + 0.2 = 0.8