In the first game:

Suppose you rolled a $6$ and a $2$.
You can make totals of $8$ or $4$ or $^-4$:

$6 + 2 (\text{or } 2 + 6) = 8$

$6 - 2 = 4$

$2 - 6 = ^-4$

In the second game:

Suppose you rolled a $3$ and a $^-1$.
You can make totals of $2$ or $4$ or $^-4$:

$3 + (^-1) = 2$

$3 - (^-1) = 4$

$^-1 - 3 = ^-4$