As the title, if it is possible to run VCC at 3.3V and AVCC at 5V?
This is not possible.Please refer to "RX66T User's Manual: Hardware" "38.6.10 Voltage Range of Analog Power Supply Pins".
Sorry, one more question. If I need to design VCC and AVCC separately, but the voltage is the same. So is there a specification for the minimum difference between AVCC and VCC?
The case of AVCC=VCC is "for A/D conversion of analog input pins AN216 and AN217".For absolute ratings of power supply and analog voltage, refer to "Table 45.2 Recommended operating conditions (1)"."Note 1: Comply with the following voltage condition: VCC_USB ≤ VCC ≤ AVCC0 = AVCC1 = AVCC2.From this explanation, if you do not use AN216 and AN217, you can make AVCC higher than VCC.For example, AVCC=4.096V and VCC=3.3V.---
Personally, I do not recommend dealing with different voltages, as there are many other things to consider.