I use this trill for baroque pieces and a very nice example is Bach's invention in Dminor tr. for two guitars. A tricky point is that when one of the two strings is open and is not the last note then it must be stopped with the right hand (for example e 1st string d 2nd string - d last note and e keeps sounding!).
When there is no open string this is much easier achieved ofcourse by lifting left hand's finger. (for example a 1st string (1) g# 2nd string (4)-g# last note, lift (1) from 1st string)
Now, wich way upwards, downwards the trill must be played and for how long is a totally different question.