WebOffice author provides in its ArcGIS toolbar a button for splitting multiple data frame MXDs to several MXDs with one data frame in each document.
ArcMap toolbar for WebOffice author - Divide an MXD into several MXD with 1 data frame in each document
It is recommended to use this function in case you have MXDs used by older WebOffice versions to prepare them for WebOffice 10.5 SP2. Let's assume that the name of the MXD document is SampleProject.mxd and it contains two data frames named "Cadaster" and "Points of Interest".

A MXD named "SampleProject" with multiple data frames
By clicking the Button Divide an MXD into Several MXD with 1 Data Frame in Each Document WebOffice 10.5 SP2 will automatically split the MXD into several MXDs with each containing only one data frame. The new MXDs will be saved in the same folder like the original MXD with the following naming convention:
•<MyMapDocument>.mxd
o<Data Frame 1>
o<Data Frame 2>
will transform into
•<MyMapDocument>_<Data Frame 1>.mxd
o<Data Frame 1>
•<MyMapDocument>_<Data Frame 2>.mxd
o<Data Frame 2>
Note: In case MXD documents with the same name already exist in the according directory, you will be asked whether you want to overwrite these files or not.
Therefore, after splitting up the original MXD, the folder will also contain a SampleProject_Cadaster.mxd containing only the data frame "Cadaster" and a SampleProject_Points of Interest.mxd with the data frame "Points of Interest".

Split MXD documents "SampleProject_Cadaster" and "SampleProject_Points of Interest" with one data frame each