I'm looking into using Atmel's 2051 microcontroller with an Xbee and a HC-05 Bluetooth module, the micro however has only one serial connection and no possibility for expandability with memory-accessed peripherals. Is this doable or do I have to choose only one of the two?
Use/find/implement your own Arduino style software-serial library for slow baud-rate serial comms to one of the devices, and use the hardware UART (serial) to the other one.
You could even do software serial for BOTH bluetooth and Xbee if data rates are kept low, and you dont have any crazy hard-realtime system (higher priority control) which may cause loss of data too often.
Yes, You can use both. You might want to look into multiplexers. You can buy dual multiplexers. Depending on the sort of IC you use, it will normally have one input to connect your microcontroller to and two outputs. Or a long way could be to build your very own perhaps using high speed transistors or even mosfets. Although using an IC is recommended