The concept is simple - different parts of your face have different needs. In my case, the lower half of my face thinks it's a 14 year old boy and the top half of my face thinks it's a rehydrated mummy which a la Cara in the Suicide Squad.
The bottom half needs gentle detoxification and pore declogging. The top half needs collagen boosting and moisture surge. Why not put them on at the same time?
The answer is no reason, none at all.
In fact, these kinds of multi-masking are gaining popularity for just this reason. One size does not fit all skin - even the skin on our own faces.
Here I am wearing a magnetic age-defying mask on the top half of my face (full review here) and a black peel off mask around the hormone zone. The peel off mask contains charcoal which is an awesome detoxifying ingredient that's also very gentle. On my lips I have a rose lip scrub, and in between the brows I added a sulfur "mask" to soothe my skin post plucking skin.
20 minutes later I was prepped and ready for date-night. (The extra time I saved I spent napping...)
Dr. Brandt Magentite Mask | Boscia Luminizing Mask | French Girl Organics Rose Lip Scrub | Malin + Goetz Overnight Acne Treatment