Per Atmel:
ATmega644:
The high-performance, low-power Atmel 8-bit AVR RISC-based microcontroller combines 64KB ISP flash memory with read-while-write capabilities, 2KB EEPROM, 4KB SRAM, 32 general purpose I/O lines, 32 general purpose working registers, real time counter, three flexible timer/counters with compare modes and PWM,
2 USARTs,
byte oriented 2-wire serial interface, 8-channel/10-bit A/D converter with optional differential input stage with programmable gain, programmable watchdog timer with internal oscillator, SPI serial port, JTAG (IEEE 1149.1 compliant) test interface for on-chip debugging, and six software selectable power saving modes. The device operates between 2.7-5.5 volts.
USART: 2
ATmega644P (Mature product; not recommended for new designs. Replaced by ATmega644PA.)
USART: 2
So why do you say one part type has 1 USART and the other has 2?