If you draw from both tanks at once what happens if for any reason one tank
loses its fuel before the other?  Think loose cap, leaking fuel line, leak
in tank for some reason. .  At this point one of your suctions is uncovered
and your pump will only suck air, which the engine will not burn.  Sound of
silence ensues and high pucker up factor is felt.

I know you have a header tank Bob but it could dramatically shorten your
radius of operation if you lose the ability to tap your wing tanks.  Guess
what, Murphy says it will only happen when you need to top up your header
tank in heavy IFR.

I hope that Bob Smiley and anyone else who can have both tanks on line also
has the capability to isolate one tank if needs be.  That is has a left,
right and both position on the fuel cock.

I may be wrong but I believe there are authorities who forbid the plumbing
of a plane to include a permanently connected "BOTH" set up.

Regards

Ian Crowe