It's better to have the same because all secondary timings will be the same, however when buying 2 different kits, the production date isn't the same and the module can still differ (but it's more unlikely vs random other ram)
DDR3 have maybe 40-50 differents timing and they should be all the same, you can get it to work for sure but it'll run at the slowest speed of the 2 kits...
PS: also getting the same looks cleaner in your build...