Yes, the standard library has many methods for splitting up a single mutable slice into multiple non-overlapping mutable slices. There's split_at_mut() which just splits at an index, or split_mut() which splits using a predicate, or chunks() which gives you an iterator over non-overlapping subslices of a given length, and more.
No, there are plenty of safe ways to achieve this in the standard library. The chunks and split families of functions on slices are all designed to do pretty much exactly this.
Everyone here is aware that split\* and chunks\* are built using unsafe. However, reaching for unsafe yourself in this situation is explicitly the wrong thing to do.
The entire point of rust's safety system is that it is possible to build safe things on unsafe foundations because the unsafety can be encapsulated into functions and types that can only be used safely. The safety of these functions then depends on them being bug-free, and the best way this is achieved is by minimizing the total amount of unsafe code in the ecosystem, and sharing it in widely used libraries so that there are enough users and testing to find the bugs.
So no, unsafe is not the mechanism GP needs to, or should use, because the split\* and chunks\* families of functions already exist and do exactly what he wants.