To make sure I understand, you have a two node ASE cluster....one of which is active, the second is idle. You want to detach (for some reason) the disks from the second node of the cluster.
From the cluster viewpoint, provided you shutdown the ASE instance on that node and don't run any cluster utilities on that node (e.g. sybcluster, quorumutil, etc.) - then whether the disks are attached or not is likely not going to be an issue. It would be no difference to the surviving node than if you shutdown the other node completely (host included).
What concerns me is that you obviously are planning something that affects that volume group - e.g. moving it to a new machine??? Swapping HW??? Updating OS volume management firmware??? If swapping HW, remember, the quorum has the list of participants in the cluster - so unless the new hardware hostname/ipaddr matches the old, you will need to modify the quorum entries (TechSupport can help with this) - although the better approach would be to build a new node of the cluster on the new HW (temporarily a 3 node cluster) and then remove the old node from the cluster. If updating OS firmware, just be careful that the patch doesn't cause problems with IO fencing by changing the SCSI PGR versions or something.
