Train in? Yes. Longer to get good? Not even close to possible. To really master a single style is the work of a lifetime. An incredibly gifted individual might master two in a single lifetime. Anything more is nothing even close to mastery - it's dabbling. Jack of all trades, master of none.
Do you consider "mastery" and "dabbling" to be the only two options?
I'm not sure what exactly would constitute "mastery" of a system. I don't claim that I have or ever will "master" a system. However after almost 35 years of training and over 10,000 hours of mat time, I think I'm past the point that most people would consider "dabbling."
At this point, I think I may view "styles" a little differently than most people. To my mind, martial art training is a tool, nothing more. Depending on the goals of the individual, it may be a tool for developing fighting ability(in various contexts), self-defense ability (in various contexts), physical and/or mental self-awareness and self-control, sportive competition, social interaction, artistic expression, or some combination of those elements. Rather than worry about whether someone is "mastering" an art, I'd rather look at how well it is working as a tool for that individual in achieving their goal(s).
Someone who is training for combative purposes may find it relevant that there are a number of young professional fighters out there who are using multiple arts to fight at a higher level than most high-ranking instructors in those arts could ever muster. Admittedly, these fighters spend a lot more hours during the week training than the average hobbyist, but they haven't spent 40 years getting to that point either.
If someone is training for personal self-development ... that's a personal journey. I don't know any way that an outsider would have grounds for saying "you would have achieved that much more self-awareness and maturity if you had trained (just one art/multiple arts)."
We talk about different arts having different principles, but ultimately every art is constrained by the exact same principles - those of physics, human anatomy, and human psychology. The apparently different principles embodied in different arts are just application of those underlying principles of reality to a particular context reflecting the judgment of the art's creators on the best trade-offs regarding pedagogy, tactical requirements, and so on. Once you get past the point of "art A does it
this way but art B does it
that way" and understand
why each art teaches what it does the way it does, then you can use either method as your current situation demands.
That said, there are good ways and bad ways to approach cross-training. This comment is long enough already, so I'll save my opinions on that subject for another post.