Based on roald’s solution (many thanks), I summarize the procedure as follows for people who might be interested.
Suppose there are two computers, A and B, that need to be sync’ed.
(1) Install Insync on A and B
(2) On A, use the “add to Insync” option to add the files/folders that need to be sync’ed.
(In nautilus: right click the file/folder --> Add to Insync --> Choose your google account. Insync will create a symlink copy inside the Insync folder.) For example, fileA is added to Insync:
A:
<somePathA>/fileA
~/Insync
fileA
Google Drive:
fileA
(3) Create a new folder, InsyncA, and move fileA there:
A:
~/Insync
/InsyncA
fileA
Google Drive:
/InsyncA
fileA
(4) Repeat Step 2 for B
B:
<somePathB>/fileB
~/Insync
fileB
Google Drive:
/InsyncA
fileA
fileB
A:
~/Insync
/InsyncA
fileA
fileB
(5) Create a new folder, InsyncB, and move fileB there:
B:
~/Insync
/InsyncA
fileA
/InsyncB
fileB
Google Drive:
/InsyncA
fileA
/InsyncB
fileB
A:
~/Insync
/InsyncA
fileA
/InsyncB
fileB
(6) On A, set “Selective Sync” to exclude “InsyncB”. And on B, set “Selective Sync” to exclude “InsyncA”.
Google Drive:
/InsyncA
fileA
/InsyncB
fileB
A:
~/Insync
/InsyncA
fileA
B:
~/Insync
/InsyncB
fileB
From then on, all new files/folders should be added to InsyncA and InsyncB folder for A and B, respectively.