The USB-C PD definitely allows you to do exactly what you suggest.

I know this is supported in this google dual-port PD charger: https://store.google.com/gb/product/usb_c_dual_port_charger

There is an explanation on how the power budget is split in this post: https://plus.google.com/+BensonLeung/posts/8jmUYtJxAwX

Presumably other multi-port chargers also do similair things