Flip-Flop triggered on the edge of two signals

Question

I need a flip flop that reacts on the edges of two different signals. Something like this:

if(rising_edge(sig1)) then
    bit <= '0';
elsif(rising_edge(sig2)) then
    bit <= '1';
end if;

Does such a flip flop exist or is there some other technique i could use? I need this to be synthesizable on a Xilinx Virtex-5 FPGA. Thanks