There are a lot of unspecified variables involved here... type of socks, fit of the shoe/boot, type of shoe, conditions you are hiking in, etc.
But in general, double-socking may offer the following which may help prevent blisters:
- reduced friction - assuming one sock is a thin slick liner sock which tends to stick to your foot while the outer sock sticks to the boot thus keeping the rubbing happening between the two.
- moisture control - a liner sock can help wick moisture away from your foot. Dry feet are happy feet.
- additional padding - can prevent hot-spots and pressure points (or can create them, depending on the fit of the shoe)
- can help take up space - shoes that fit too big can rub more
- leech control - two pairs of tight woven socks have served me well for 2 years in the tropics
- warmth - adds additional insulation (though if your boot is a tight fit, this can actually work against you as you decrease circulation/air space...)
If the question is whether double-socking is better, that is unanswerable and totally a question of personal preference. If 2 socks work for you, and reduce blisters, go for it. For other people's feet / shoes, 1 sock may be better. I've even worn 3 socks when my shoes were big enough, and I have hiked with people who wore no socks without issue...
If the question is whether two socks are proven to reduce friction - even that depends on the fabric of the socks, the material and fit of the boot, the foot, and the environment (hot, cold, wet, dry, etc...).