There are some calculations whether second monster will appear or not and it depends on how many levels are there between you and your enemy. So it's perfectly normal for second one in some cases not to appear.

they are lvl 42 like me tho

Kingboxa,

If i remember correctly for second to appear difference needs to be either four or five levels between you and the monster you're fighting.

just tried lvl 41 sinra slave and 2nd spawned whats goin on?

Everything has been cleared up within the game, level 42 protector quest puts you up against Level 45 Sinra Hangman's which makes difference between them and your dragon of five levels so therefor there were no second monster spawned.