UART: interfacing b/w 3.3V and 2.4V - current limiting resistor
First, check the current sinking capability of gpio of IC 2.4v and current sourcing capability of gpio of IC 3.3v.

If sourcing current of the IC is lesser then sinking, then there shouldn't be any problem. But if it is higher then, u need to calculate the resistor value which will limit the sourcing current towards sinking. A simple ohms law will do for calculation of the resistor. 

Also, check whether the IC 2.4v pins are tolerant to 3.3v. If it is, it will simplify the solution.