Tom,
Technically, it is possible to run multiple clients on the same physical machine (i.e., OS), however, this can quickly become a complex and frustrating environment to maintain for more than a few tiles. This is because the ports used for WORKLOAD_CLIENTS[Tile x][ Workload y], and the CLIENT_APP[Tile x][workload y] must all be unique. I suspect this may be the problem you are encountering. Most folks find using virtual clients (which can be cloned, replicated, and then edited only for /etc/hostname numerical IPs) a much simpler implementation.