Combining characters using their hexa name

The \c[…] escape is for declaring a character by its name or an alias.
0306 is not a name, it is the ordinal/codepoint of a character.

The \x[…] escape is for declaring a character by its hexadecimal ordinal.

say "\x[0306, 0307]"; # OUTPUT: «̆̇␤»

(Hint: There is an x in a hexadecimal literal 0x0306)

\c uses decimal numbers:

say "\c[774, 775]"

where 774 is the decimal equivalent of 0306, works perfectly.