Function vcombine_s64 

pub fn vcombine_s64(a: int64x1_t, b: int64x1_t) -> int64x2_t

Available on (AArch64 or ARM64EC) and (AArch64 or ARM64EC or target feature v7) and (ARM or AArch64 or ARM64EC) and target feature neon only.

Join two smaller vectors into a single larger vector Arm’s documentation