Your phrasing shines light on a problem some people had with earlier editions, and 5E's approach to handling it (as I understand things): PCs could apply multiple modifiers (buff/debuff/control spells) without limit, risk, or tradeoff, so in 5E let's shut it down completely—per character, at any rate. I understand the reasoning, but the obviously "gamist" approach to concentration is what has always irked me to bits. At least give me some motivation for why a completely inert wall of stone needs me to concentrate on it!
Adding force mulipliers could have been done with escalating cost or risk. I'm currently in a Torg Eternity campaign, and for every concentration power you have active, you accumulate a -2 to all spellcasting (you have to make a skill check to cast a spell in Torg), and if you blow the roll, all your concentration spells go poof. So it's possible to have 2 or more concentration effects if you want, but you're taking a risk! Torg is definitely on the other side of this question with regard to force multiplers, and things can get very gonzo indeed. But this thread is not about Torg, so onward!
Now, 5E also has a goal of reducing numeric modifiers in favor of the nice & simple advantage/disadvantage within a system of bounded accuracy, so boundless stacking isn't viable, but I just made a suggestion last night in another thread about this:
It's just a spitball idea, and as with all such things, potentially a first step on a slippery slope. Concentration spells are currently tuned for just one per character, too. But, this change might work for allowing some stacking of force multipliers, and make
this aspect of combat more fun for
some people. Of course, it might make this aspect of combat
less fun for some people too. Best not to even consider it! #nochanges