@mike-cornelison, like Xander wrote, you can use & between your commands to run multiple commands on one command line.
But you said you're running a batch file. So couldn't you just put multiple lines in that batch file? Like so?
start http://127.0.0.1:8086/?x10command=DEVICE~sendrf~%%27A3%%20ON%%27
start http://127.0.0.1:8086/?x10command=DEVICE~sendrf~%%27A4%%20ON%%27