So the way I usually do this is to drive all the client tests from a
single Flent instance. There are two ways to do this: Run Flent on the
server and connect to each client, running the test "in reverse", so to
speak - the --swap-up-down parameter is meant specifically for this.

The alternative is to use the remote execution features of Flent to
start the test binaries on the clients using ssh (works best using a
dedicated control network). This is what the --remote-host parameter is
for.

The benefit of driving everything in one test is that this way Flent
also takes care of synchronising the data series into a single run
(assuming clocks are reasonably synchronised across machines when using
--remote-host), and you'll get everything in one data file. It can be a
bit fiddly to set up, but you can encode everything into a single batch
file so repetitions are easy...