(I suppose you are talking of two RM32AI.) You need an AVB switch, a router and a computer connected by firewire. The RM's have to be connected to the AVB switch by CAT5 cables; for the router there is a special port. After you have got it all set up, start the router and let it boot fully, then start the RM32 which you want to use as master, start the second RM and then start your computer with firewire connection. Now start Universal Control and you should see one RM connected by firewire and one by WLAN. Click on the mixer with the firewire icon to open UC Surface. Then go to the firewire mixer's settings (networking) and make your master RM connect to the slave RM. This takes some time (about 15 seconds). After cascading you now see 64 channels as one mixer in UC Surface. Then start Capture. You now see 64 plus 4 tracks armed and ready to go. Remember that mix outputs are not cascaded, but merched. The slave RM's outputs mirror the master RM's outputs.
This is how you DO it.