I2C potentiometer mess
The registers are listed in table 4-1. Clearly enough.

The command byte format is shown in chapter 6. That command byte includes the operation and the register number, as well as upper bits for registers that expect more than 8 bits of data.

Two bytes are needed to set a register.  The first 4 bits are the register number (0000 for wiper 0, 0001 for wiper 1, and 0100 for the TCON register).  The next two bits are the operation as shown in figure 7-1. 00 for a simple write. The next two bits are the upper 2 bits of the data.  Following that are the lower 8 bits of data.

So to write a pot 0 value of 0x23 you would write:

0x00 0x23


To write a pot 1 value of 0x65 you would write: 

0x10 0x65


To write a pot 1 value of 0x13D you would write:

0x11 0x3D


You can make a simpler function to wrap it for you:

void writePot(uint8_t address, uint8_t pot, uint16_t val) {
    Wire.beginTransmission(address);
    Wire.write((pot &amp; 3) &lt;&lt; 4 | ((val &gt;&gt; 8) &amp; 3));
    Wire.write(val &amp; 0xFF);
    Wire.endTransmission();
}