Basically, AC450 has wide range of data conversion FBs [ex. CONV-IB, CONV-BI, CONV(IL,R), etc.].
You may convert each of the double integer 2 word (ex. 16 bit) to binary using CONV-IB(I,4,16); Then merge the 16 binary output of each FB to long integer using CONV-BI(IL,4,31); Then convert the long integer output of the FB to real using CONV(IL,R).
According to the data type & format received over MODBUS, you may decide the correct parameters of the FB. For further assistance, it is better to share printout of the MODBUS program at AC450 showing double Integer data received.