If I had 0.5mol dm-3 solution of sodium dihydrogen phosphate and 0.5mol dm-3 of disodium hydrogen phosphate and mixed together 10cm3 of sodium dihydrogen phosphate and 50cm3 of disodium hydrogen phosphate what would the concentration of each solution be?

C1 x V1 = C2 x V2
concentration1 times volume1 = concentration2 times volume2

That's the equation you want to rearrange, just work them out separately and remember to watch your units - there is dm3 and cm3 here.

EDIT
you might think you don't have volume1 you do- look at the units

If we mix two acidic or two basic solutions then final concentration=(c1v1+c2v2)/(v1+v2)

If we mix one acidic and one basic solution then
Final concentration=(c1v1-c2v2)/(v1+v2)
[here c1v1 is taken as concentration*volume which ever is more should be taken c1v1 i.e final conc. Is positive]

Use the above results according to your need

If we mix two acidic or two basic solutions then final concentration=(c1v1+c2v2)/(v1+v2)

If we mix one acidic and one basic solution then
Final concentration=(c1v1-c2v2)/(v1+v2)
[here c1v1 is taken as concentration*volume which ever is more should be taken c1v1 i.e final conc. Is positive]

Use the above results according to your need