I agree with your points 

The reason why I came up such a question is that , my roommate who was learning German and preparing for CET-6(College English Test in China) a few day ago , and when the test was over , it suddenly occurred to him that he had written "ist" in the whole test where should have been "is", so he felt very frustrated and complained to me...

Thanks for your discussion , I'll have him know this

The side effect I see (most of the time) is that the people are no able to dedicate enough time for the multiples choices, so they don't learn neither.

I'm also learning two languages at the same time. I don't see any side effect, maybe because I know those languages on different levels...

But I also noticed that many people are learning a lot of languages at the same time and I don't see a point in starting 4 new languages at the same time. For me it won't be effective learning ;-).

Hi Nick,

it is possible to learn two languages at the same time, as a matter of fact I studied English and French at the University.

There was no side effect because these two languages are very different.

I’m 100% with Gil. If you split your time, you’ll slow down progress, possibly below a critical point. I’ve done it (and continue to do so), but is a lot more efficient to bring one language at a time to an advanced level.

As far as “interference,” like writing “ist,” that can usually be avoided by using appropriate learning methods, which we should be using in any event. As far as your friend’s test, hopefully they only counted it as one mistake. In the real world, no one would even care.