They are not all that different until two particles or two strings are involved. In quantum mechanics, "either one string is vibrating or the other" is a state

They are not all that different until two particles or two strings are involved. In quantum mechanics, "either one string is vibrating or the other" is a state the strings can be in, and a state that can be added or subtracted from other states. In classical mechanics you can say "half of the energy is in a low frequency vibration and half is in a high frequency vibration," which is roughly equivalent (mathematically, not conceptually) to a single particle either being in a high or low state - but as for two strings, either one is vibrating or the other, and you might not know which one is vibrating, but one is.