Generally I think it's a bad idea.
You should use only one browser during test. If you need to emulate another user's action, do it "technically", not via UI. Say, update data in database or call some API methods.
If you still want to do it via UI (which is bad idea because UI is slow and unstable), you can create new webdrivers manually and set them via method WebDriverRunner.setWebDriver(); Something like this:
WebDriver wd1 = new XXDriver();
WebDriverRunner.setWebDriver(wd1);
/// actions 1
WebDriver wd2 = new XXDriver();
WebDriverRunner.setWebDriver(wd2);
/// actions 2
wd2.close();
WebDriverRunner.setWebDriver(wd1);
/// actions 3
wd1.close();