Gideon will be able to answer more appropriately if this is really possible, but Q2A is fairly modularised with all the database stuff in one of two files/classes, so I would guess it is possible.
However, it's absolutely not necessary. 20 users accessing the system at once is NOTHING. Apache allows up to 150 simultaneous connections by default, but even if 150 people are browsing the site at the same time you probably won't even have 10 simultaneous connections since each lasts under a second.
PHP and MySQL are both very fast and Q2A is designed to run fast as well. Gideon also said here that he's tested the system against a large database.