Best practice is to keep Azure Monitor and Sentinel separate but this is largely a preference. Many will argue separation of duties, data isolation, and cost savings. The same agent can be used to dual-home to both workspaces. Caution should be taken to avoid data duplication.
Using the same workspace can be very convenient, especially for smaller IT teams where security and operational support may overlap. Sentinel will nearly double the price initially. Keeping data separated will save money but if the operational data is reasonably sized and convenience is preferred the consolidation may make sense. You can even create alert rules in Sentinel that are more operational if preferred without the pre-rule charges from Azure Monitor.