You can with btrfs. Shrink down your filesystem with btrfs filesystem resize (it can be used on a live session), then shrink down the partition with parted. It seems a little like open heart surgery to do this while you are booted in to it, but I have done it many times and it’s fine.
From there, parted again to add a new partition (we’ll call it p3) after the one you are booted into (p2), then btrfs replace to move p2 data to p3. Again, due to btrfs awesomeness this can be done while you are booted into it.
That leaves p2 empty. You can delete the partition and use parted to expand p1 or p3 (or both), or keep p2, or whatever you want. Don’t forget to btrfs filesystem resize max after you are finished messing around, so the filesystem will fill up the partition again.