plug 5V serial devise with a due

HELLO EVERYBODY

i would like to know if it's possible to connect a 5V serial devise like a simcom sim900 and a adafruit ultimate GPS with the DUE.

i dont want to power the serial devise with the board (for that i use a 5v lithium bat)
i just want to plug the TX and RX pin together.

if its impossible...

i also have a UNO, but there is only on TX RX pin on this one.
Do you tink i can be able to plug both of this devise on the same uno card.

i would like to make a kind on ELT(emergency locator transmitter)
when an impact happen, an accelerometer will start the gps.
after that; the gms module will send my position to a cellphone

tanks for your help