Not sure about plugging in two chargers at once, but you can definitely use either MagSafe or USB-C to charge. I do it frequently with my MBP. The MagSafe charges faster, but that may be more down to the power brick than the interface, I don’t know.

The new 2021 MBPs (14 & 16) as well as the new M2 MBA/MBPs can charge with either Magsafe, OR USB-C.

I've personally charged my 14" off a 100W USB-C, and it does fast charge, faster than the stock 67W adapter.

mag safe is how to get the 67w, usb c will be slower but possible. This i researched too because wanted a USB-C Exclusive EDC and it looks like thats possible albeit slightly slower charging which is fine if you don't rely on 100% battery dumps then full fast charges to operate.

Wrong. It depends on the USB-C charger/cable; it can and will charge at >67W via USB-C if your wall adapter & cable can handle it.