Sequencers only work by "reacting" to the presence of a binary property of a certain type: txt, mp3, images etc. All the sequencers expect this property and parse its content in various way.
Nodes are something else altogether, so it doesn't make sense to trigger any sequencing based on the presence/absence of nodes.
What is your use case ?
I have two sequencers, A and B that execute on the upload of a single file.
They create trees under /a and /b respectively.
They take 1minute and 5minutes to execute.
Once they have executed, a third sequencer, C executes to update node references in /b back to /a.
Sequencer C watches /a and /b waiting for both to exist.
I could add a property to /a and /b just for this, but it seems like a hack.
We do have an extension to the JCR event mechanism, allowing you to listen to sequencing events. So C in your case can be just a regular listener which after it receives both sequencing events does what it's supposed to do.
See Sequencing - ModeShape 3 - Project Documentation Editor and modeshape/AbstractSequencerTest.java at 3.x · ModeShape/modeshape · GitHub for the way in which you can use such a listener.