I think I read somewhere a clarification on MuVi guidelines but could'nt find it back.
Page 159, it says that you cannot cast the MuVi after the one you alter.
It is not specified that MuVi spell can or should be cast before the other one and looks like one can cast MuVi spells affecting it own spells.
("Shroud magic" seems to be the exception, as it is the only one that can be cast later on)
Are they both cast at once or in sequence ? (and wanyway requiring the Intelligence + Concentartion roll of 9+)
Are there other spells you can cast both at the same time ?
(note : posted on Berklist too)