Can think of more negatives than pluses ...
for one, very complicated config file for 2 sites - have seen such a set up for 4 sites.
updates ... which come frequent enough that updating the DB for each site becomes 'an adventure' ... and actually takes longer + more prone to human error.
upgrades ... even more prone to human error ...
Neither of those should be using the GUI/Web based updates or upgrades.
How does one test plugins???
So yeah could have one code base ... 2 moodledata directories ... 1 DB **if** one site uses Amdl_ prefix and the other site uses Bmdl_ prefix.
Oh, yeah ... let's talk about that single database ... logstore_standard_log table for one site gets in the 2 Gig range farily quickly .. X 2 ... 4 Gig's ... just for the 'who done it' table. So memory usage of server (DB memory) will need to be monitored and increased more frequently. Of course if you had a dedicated DB server with plenty of RAM to begin with ... no problem.
So again ... why?