ADF pipelines will execute the notebooks as follows:
if you use a dedicated cluster and you run 2 notebooks simultaneously on the dedicated cluster, it will acutally run both.
Mind that dedicated clusters are more expensive than job clusters.
The same can be achieved using notebook workflow where you call parallel notebooks within one single notebook (which is scheduled in ADF). So like that you do not have to use a dedicated cluster and instead use a job cluster.
The main downside of this is that your cluster may get hammered because of the parallel runs. Not necessarily, but that is definitely a concern.
So you could also opt for a cluster pool, whic you can use in ADF. It is not exactly the same as using a single cluster but workers that are not needed can be used for other jobs, until they timeout after x minutes of inactivity.
So depending on your scenario you can go one way or another.
Me, I never use dedicated clusters because of the price. So I use separate job clusters, notebook workflows and pools.