I think if you have two producers you should make them connected to different nodes.
As you ramp up the number of clients sending and receiving messages then at some point the throuput of a particular producer/consumer will slow, so no you won't get Linear Scale (although for a small number of clients you may at first). Throughput is typically restricted by the speed of your hard disc (if you are using persistent messages or transactions) and the speed of your network. Other things that wlll have an effect are paging (it may have kicked in), acknowledge mode, number of threads available to server, distribution of clients around the cluster etc etc.
And Howard is correct, for optimim perf you should distribute your producers