cast only on cooldown?

I would like to do something like this…

#showtooltip Immolate
/cast Dark Soul: Instability
/cast Immolate

but in WoW it will just stop

even /castsequence Dark Soul: instability won;t work

I am looking to cast Dark Soul only on cooldown and then cast Immolate the rest of the time

any help is appreciated



assuming you are using gse

ive put mine in the Premacro box with an innerloop limit of 2
just using:
/cast [nochanneling] Dark Soul: Instability

and it works like a charm