What if the level signals of two UARTs mismatch?
You want to use a level converter to go back and forth between the 1.8V level and 3.3V levels.  SparkFun has three of them that will work; I have used the one based on the TI TXB0104 chip.

There are four circuits; you only need two of them. You'll want to cross-connect the TX and RX leads; i.e. connect the RX lead of one UART to the TX lead of the other, and vice-versa.



You can also build your own converter using MOSFET's; in fact SparkFun sells one like that also, the schematic is here.