it's ok to use more than 1 agent for a group, in fact if 1 agent can't support the load of a specific group, then we would need an additional agent or even more. Usually we know we need another agent if we get a message similar to "the workbench has lost communication with the agent on host xxxx'
Same for Many "User Groups" using the same agent.
Starting with RPT 9.5, datasets aren't allocated on a machine (segmented) basis.
Dataset row requests are allocated on a first come first serve basis in the order any given VU requests a row, assuming the access is sequential.
Please refer to this technote for all the open modes and how they behave:
"Understanding the Various Open Modes for Datasets in Rational Performance Tester"
https://www.ibm.com/support/pages/node/1076889
#RationalPerformanceTester#SupportMigration#Support