המשנה בתרומה אומרת, המפריש מקצת תרומות ומעשרות מוציא ממנו תרומה עליו אבל לא למקום אחר. ר' מאיר אומר אף מוציא ממנו תרומה על מקום אחר.

The Mishna in tractate Trumot says when one separates  truma and maasar {1/10} in part, he must separate truma from it but not for another place. R. Meir says also truma for another place.

It seems to me that if this mishna would only be talking about maasar it would be simple. And if it was only talking about truma, it would also be simple. What makes it difficult to understand is the fact that it puts both together.
Let's say we were only talking about Maasar. And instead of his separating 1/10, he separated 1/20. And we have the principle אין ברירה. (No choice). [That principle says for example if two children of an idolater inherit is wealth. But one son became a convert.  If we would say יש ברירה (there is choice), then he could tell the other son you can take the idols and I will take the other property.  And by that it would be revealed that even at first, the idols fell to the portion of the idolatrous son and the convert could take his own portion. ] But if there is not retrograde choice. then he could not do so. He would have to take an even portion with the other son, and then the idols that fall to his portion he would have to destroy.

So in our case also. If we would say יש ברירה then it would be simple to say he simply takes another 1/20 and the stack of grain is fixed. If אין בררה then it is clear that  the stack of grain is mixed with טבל and חולין.
If we were only talking about Truma, then also it would be perfectly clear. The law of the Torah is  is  אפילו חיטה אחת פוטרת כל הכרי
Even one grain fixes the entire stack.