It depends on the solution.

If you have 1L of hydrogen peroxide.

Plus

1L of Copper.

**Your total should yield 2 liters.**

And, the final concentration would be essentially the average concentration of the two solutions you added (half of what you calculated).

If you are making a single combined solution, remember, one first adds the two solutes, then brings the solution up to precisely 1L.

If you are dealing with milligrams, then it is often easiest to report the answer in millimoles.