Sorry, it is not clear what you are asking… As I read it you think of a SL board as FOH and a RM as "stagebox". This needs an AVB network connection. The RM can then work in "stagebox mode" or in "I/O mode". The first one can be fully controlled by SL, the second one needs UC Surface on iPad or computer. Cascading SL boards is handled only by Firewire, whereas RM's are cascaded only by AVB. Due to this constructional difference you can not "cross cascade" SL and RM. As an alternative two SL boards can also be part of an AVB network, but they remain being totally independend mixers. You will see two riders in UC Surface and you can "switch" between the boards.