Actually there is a "bug" that has been around for a while where it's not possible to fit pet abilities in a castsequence macro. There are ways around it (I am using this trick while flag carrying in battlegrounds).
This will keep it under one button, however through the use of a modifier (shift/ctrl/alt).
/cast [mod:shift] Shadow Bulwark (special ability); sacrificial pact
Remember though, activating Shadow Bulwark only increases the voidies health pool, but not its actual health. This means if you spam sacrificial pact right after shadow bulwark, the effect is very minimal (as the shield is 50% of his remaining hp. I myself use glyphed health funnel to top him off before I hit my shield. This can be done with a castsequence
Reason I don't bake this in the previous macro is that if you're silenced, you won't be able to trigger sacrificial pact (as it wants to cast health funnel first)
/castsequence reset=2 health funnel, sacrificial pact
Hope it helps,