The first kit adds 16 balanced tape outs ( these are in parallel because the 3500 is a 8 buss board ) and 16 balanced tape ins. The second kit adds 16 more balanced tape outs and you discard the other 16 tape outs.
1 kit = 16 balanced tape outs ( parallel) and 16 balanced tape ins.
2 kits = 16 balanced tape outs ( parallel) and 32 balanced tape ins.