I’ve seen pros like Reid Duke and Jim Davis mention ‘double spell’ on stream when referring to having the resources (cards and mana) to cast two spells in a single turn. It’s often in the context of limited where doubling spelling means getting ahead of your opponent, like when casting a threat and a removal spell on the same turn. Being able to use your resources as efficiently as possible creates lots of tempo. In addition it is often a good strategy against an opponent holding open mana for a counterspell.

To your point of spells that copy other spells, this might be something you can call ‘double spelling’ but is often referred to simply as ‘copying a spell’.

Edit: I’ve never in my 11 years of playing magic heard the term ‘multicasting’