A short answer: No, this is not possible.
A longer answer: As you are speaking of the Series III 16R rack mixer, it is very important to realize that this is the ONLY 16 channel and single core mixer in the whole product line. There is one quad-core 64 channel mixer, the 64S; all other mixer are 32 channel dual-core mixers. This means that whatever you "add", it will always stay in its basic channel = track confiuguration.
In other words: Two 16R in an AVB network (can be connected directly) will NEVER have more than a 16+2 track USB recording. AVB recording would need a direct connection between computer (Mac only) and mixer, which makes a connection to a second mixer impossible. This is an either or situation. The reason for the limit of 16+2 tracks is that USB can "see" only the 16 tracks/channels coming from the mixer it is connected to. Therefore two 18x18 USB units can not be "added". It is a totally different situation when a 24R or 32R rack mixer is the interface for recordings. In this situation a 16R can "add" 8 channels to the 24R via AVB.
And one last thing: Only the StudioLive 64 is a true 64x64 USB interface. The 32 channel mixers are not able to record more than 32+2+2 tracks via USB.
