How to use two NXT bricks for 3+ motors and 4+ sensors?
Maybe you should look into using a multiplexer to add additional sensors to an NXT brick.

The Hitechnic Sensor Multiplexer lets you connect 4 sensors using only 1 of the NXT connection.



The same thing can be done with motors. For example using the Mindsensors NXTMMX, you can add 2 additional motors per multiplexer (even better, if you daisy-chain the multiplexers together you can control more motors than you'll ever need)