Well, I just did some homework with Amurph's lesson in this thread for engaging two targets with manual TDC. Long story short, I fired 3 fish on slow speed at the front merchant when he was at AoB=0, then swung the scope to the following merchant and, after putting his identity in the TDC, I ranged him, entered new AoB and fired 3 fish at fast speed. As torpedo 4, 5, and 6 were leaving their tubes, numbers 1,2, and 3 were hitting the front ship. The second ship didn't have enough time to maneuver before all three fish hit him as well.
This thread has been most usefull. Thanks!