Understanding How Double Precision Numbers are Stored in a Computer
I don't know how old your book is, but some architectures (I know of the early SPARCs, there may be others) only had 32 bit floating point registers. Double precision instructions used pairs of registers for storage. This became much less common on general-purpose hardware when 64-bit integer registers became the norm.

If anything, we have the converse today, thanks to vector/SIMD operations. If you have, say, a 128 bit vector register, you can fit four single-precision or two double-precision numbers in it.