What is the exact expression you were using? Do you have the high and low words brought in as OPC tags already, and you're now just creating a 3rd expression tag to combine them?
P.S. the float case is tricky and can't be done with the expression language alone: 64-Bit Double Little Endian Input Register - #25 by Kevin.Herron (there's also a corresponding Float.intBitsToFloat in the Java standard library)