DS81 is a pure diseqc 1.1 switch, as far as I know.
I don't know the brand and type number of the other switch.
If both are diseqc 1.1, they cannot be stacked, as they switch on the same commands.
You'd need a diseqc 1.1 UC4 switch to switch between two 8/1 diseqc 1./1 switches.
Another simple...