Actually, after a lot of reading I tried to master the multi male orgasm technic. Basically (but may by I was not doing it well), a bit before cuming, I was stopping any activity, tightening my verge and focusing very strongly on the feeling to make it internal.
I managed to do it but the result was a bit disturbing.
First a very small amount of sperm was still going it without any actual ejaculation. I was not really feeling pleasure of satisfaction. I was still hard for another session, but not as hard as before. Last but not least, it was taking me another twenty to thirty minutes to actually cum, or a total of thirty to forty minutes.... Which is by any standards way too long. As I am quite physical in bed, it turns to be quite tiring and not exactly enjoying.