hard disk combine

Ive been trying to find the answer to how to combine two disks and i just deleted a volume in disk management while watching a tutorial and the tutorial said that i need to create a "New spanned volume" and i only have the option to create a "New simple volume" and ive looked this up and people...

Let's say hypothetically i have 2 HDD , one is faster than the other .If i merge them into one Local disk , what happens with the speed of the both ? Im wondering , for example i download something which is 120gb into the merged disk and then i decide to copy it to somewhere else and because of...