Can we reduce an NP to an NP Problem?
It depends on the problems. Just knowing that $A$ and $B$ are in $NP$ doesn't tell you whether $A$ is reducible to $B$.

If $B$ is in $P$ and $A$ is $NP$-complete, then $A$ cannot be reduced to $B$ (unless $P=NP$).

If $A$ is in $P$ then it can be reduced to any problem $B$ except $\emptyset$ and $\Sigma^*$.

If $B$ is $NP$-complete then any problem (in $NP$) can be reduced to it.