The first time you try it, you might want to test them to make sure that they are actually "isolated" types. I think you could do that by connecting a 10k resistor (say 5k or higher, if using 1/4 Watt resistors) between the 24V output of one and the 0V output of the other one, and then measuring the voltage across the resistor. If it's zero Volts, then they should be OK to connect.