Mixing box bleaches?

You mean bleach kits?

Same brand different strengths, do you mean something very close like Feria Absolute Platinum and Feria Very Platinum? Technically you CAN mix them, but be very careful with box bleach because you don't know what strength developer they have. You can even use the powder bleach with a separate, lower volume developer, if you'd like, to minimize damage. I cannot be 100% sure about mixing whatever brand you have since I don't know what it is, so I'd wait for a second opinion before doing it.

Yeah, I got 2 Schwarzkopf kits, but one's an 00A and the other a 00B and I don't quite know what the difference is :/

I've used them both.  00A has more bleach powder in it than 00B, and lifts my hair about a full shade lighter than 00B.

I personally wouldn't mix them together.  If you're lightening your roots to put another colour over it, it should be fine to do your top and crown 00A and the back of your head with 00B.  Other than that, bring back whichever one you don't want to use and exchange it.

Great advice! I used both but didn't mix them, like you said I used the 00B on my roots and crown and then the 00A on my lengths and I've gotten to the colour I wanted, I'ma let it rest for a few weeks before I attempt anything else with it though. Thank you again!