Hi!
Let's say Folder 1 is your first skin and Folder 2 is your second skin.
Open your osu!file location and head over to your Skins folder.
Create a new folder to whatever you like; I name it Folder 3 as shown below.
With that, you just need to copy the elements that you would like from Folder 1 and Folder 2 into Folder 3 - and reload your skin by pressing "Ctrl", "Alt", "Shift", "S" keys altogether.
Be sure you don't missed any filename available in both skins, otherwise it will shown default.