A common way to address the individual bytes of a word (or larger)
variable is to split it as an array of bytes.

This can be done as follows

var word w
var byte wb[2] at w

You can then refer the two bytes of the word w as wb[0] and wb[1]

If the individual bytes of the array wb[0] and wb[1] need a specific name alias keyword can be used as given below.

alias hibyte is wb[0]
alias lowbyte is wb[1]