Adding summing to existing setup

Hi guy’s, fingers crossed someone can help clear my head. I have one MR816csx and all the imput’s are used by my hardware synths and beat boxes and the 8 channels of adat goes out to a Presonus DP88 which feeds a tone amp and bus compressor. Can i add two more mr816’s chained by firewire to the existing MR816 and have 16 additional channels of summing through the analogue imput’s and outputs. Is it that simple or will i run into problems.