If you do not really need concurrent, simultaneous access, I would use the
data source to create a directory type mailmerge that you merge to a new
document (the only possible destination) and then save that document in a
location to which you both have access.

The directory type mail merge main document should contain just a one row
table into the cells of which you insert the merge fields from you current
data source.  When you execute that merge, the resulting document will
contain a row of data for each record in the data source.  You can then
insert a row at the top of the document and into the cells of that row, you
can insert the names of the merge fields (not the merge fields themselves,
just their names).  If you save that document, you can use it as a data
source.

Note that only one of you will be able to access and use the data source at
the one time, but you will both be able to use it at different times.