In my youth (up to about age 30) I would often cum three times in relative quick succession, say over a period of 60 to 90 minutes, without ever going soft. But that only happened while fucking a gal, never had that experience while masturbating. To this day I can still cum three or four times over a 24 hour period, but going soft and resting between orgasms.
Once, and only once in my lifetime, I had two strong ejaculations within minutes of each other. That was shortly after I turned 50 and I had been so busy at work that week that I had only whacked off briefly each night before going to bed. But come Saturday I was jacking off and edging out in the sun in my back yard (I have no nearby neighbors) and I could feel myself getting really, really close to cumming. I was about to back off and continue the edging but I went over the brink and shot a huge load of 8 or 9 squirts. I continued to slowly stroke my dick, as I usually do after an orgasm, but that time it felt very different. I could sense the masturbatory arousal it was causing and my dick got even harder instead of starting to go soft. So I sped up my stroking and within a minute I got that sense of undeniable certainty that another orgasm was on tap. So I beat my meat furiously for about ten or fifteen seconds and shot another load as big as the first. That totally amazed me, I had no idea the ejaculatory ducts could reload that fast. My prostate and seminal vesicles must have been loaded to the max when I started that session. In the decades since I hit puberty I know I've had tens of thousands of orgasms but there are only a handful of them that I remember vividly. That double shot out in the back yard ranks high up on the short list of those memories.