Question keyboard input problem on 2 keyboard input at the same time,

[pekto2]

hi guys! i have a project where i need to create a GUI version of a quadcopter remote control, i already build it hardware using AVR MCU from atmel,
i was able to send string through UART using the KEYDOWN and KEYUP event, but when i need to press ARROW-UP and ARROW-left on keyboard simultaneaously
the last KEYDOWN event is prioritized by the form, i did make a code for combination keypress to give the quadcopter a diagonal movement by sending a single string through UART. can you give me tips and advice..all i need i to send a string to my AVR MCU. THANKS IN ADVANCE!! =D