And the reply is, because if you do that, then you are being a test
pilot.
Really nice answer !
I'll use it meanwhile...

At any rate, I would certainly not be willing to find out if it works.
But if you really need to know, can't you just look at the fuel system
diagram in the POH?

That's what I did. In the diagram, as it should be, there is no
connection between the two pipelines. So, as far as the diagram
reports, no problem should arise.
Also.. since in short taxying like those in small fields you don't
have enough time to test both X-FEED pipelines each at a time, I heard
is a common beheaviour to test both at the same time.

I am going to actually look at the valves operated by selectors.
Maybe the two valves are mounted nearby and there is no enough room
for them to operate properly at the same time... it's just a
supposition.
I'll tell you about.

Silvio