One-taped RAM vs Multi-taped RAM
Random-access machines support the following operation in constant time:
$$
x \gets M[y],
$$
where $M$ is the memory array, and $y$ is an index whose allowable size depends on the exact model. Whether $M$ is an array of bits or an array of words depends on your exact model.

If you had several different memory arrays, say $M_1[y],\ldots,M_r[y]$, then you could simulate the command
$$
x \gets M_i[y]
$$
with the command
$$
x \gets M[ry + i]
$$
which also takes constant time.

Therefore there is nothing to be gained by allowing several "memories".