It's not that they're in more than one tier at the same time, it's that those are different tiers for different forms or time skips. For instance: Maybe one character in the beginning of the series is 9-B (Wall level), but they become stronger to the point where they can destroy a building later on in the series (8-C).
To categorize these, we use "Keys". This basically just means separating characters based on where they were in that point in time, what form or transformation boosts they got, or something along those lines.
Example: Let's take the first example I gave about the 9-B and 8-C. The first one would be the "Beginning of the series" key, and the second one would be the "Time-skip" key. Those would be categorized like this:
Tier: 9-B | 8-C
The slash in the middle is to represent that they're different. One is for the beginning of the series, and the other is for a time-skip. Don't forget to specify that these are those types of keys. To specify, just write this down somewhere:
Key: Beginning of the Series | Post Time-Skip
You can also do this with forms that would change the power level of the character. For instance: Goku would have multiple levels of power, and would get stronger with each form, so we can categorize him like this:
Key: Base | Super Sayian | Super Sayian 2 | Super Sayian 3 | Super Sayian God | Super Sayian Blue | Ultra Instinct Omen | Mastered Ultra Instinct
Then after that all you'd need to do is add the respective tiers and that's it.
TL;DR: They're not more than one tier at the same time, but instead split apart in either different points in time, or different forms.
Hope that made sense. ^^;
