Is it fair (as in rational, smart, non-stupid) to put
multiple 7805 in
parallel? Can't say I've ever seen that done. Only the more typical
design of adding a pass transistor to carry more current.
It's a bad idea to parallel regualtors like that, since the output
votlages will not be indentical and one will supply the lion's share of
the current (and will try to feed current back into the others).
What you can do is have separae regualtors for different bits of the
circuit (the fact that all the 5V rails are not _quite_ the same won't
matter). The S100 bus did this, of course (regulators on each card, the
5V line was not bussed between cards). I believe very early versions of
the BBC Micro PSU (the linear one that 'took the paint of tables' because
it ran so hot) used 3 7805s with 3 feed wires to the logic board. On
later boards, designed for use with the SMPSU, the 5V terminals were all
linked, I beliueve they may not have been on very early boards.