They are definitely different!  You do wind up with different response
curve - by reason of the different exposure times at the two grades, if
nothing else.

I use split printing when I want the tonality of a lower grade, but the
blacks just aren't there.   Often a problem with underexposed
negatives.

First, do a test strip on the shadows with a grade 5. Pick the interval
just before the deep shadows become true black. (Grade 5 Exposure)

Then expose a strip on grade 5 at the Grade 5 Exposure.  Then switch to
a grade 0 or 00 filter and (on the same strip) do a test as per normal.
Pick the exposure that which shows the detail in the highlights (Grade
0 Exposure).

Then do you test print - first with the grade 5 filer at the Grade 5
Exposure, then with grade 0 (or 00) at the Grade 0 Exposure. Hold the
filters under the enlarger lens (instead of placing them in the drawer)
to avoid any enlarger movement.

You may have to then tweak the Grade 5 Exposure and Grade 0 Exposure.
If the shadows are blocked up, back off a little on the Grade 5
Exposure.  If the highlights aren't there quite, increase the Grade 0
Exposure.

The Grade 0 exposure time might be quite short.

Split printing doesn't work for everything, but is very useful for many
underexposed negatives with weak blacks - especially twilight and night
shots.  Other alternatives to split printing include dodging and
burning (of course); simply printing on a high grade (which gives you
separation of close tones & perhaps burning on a low grade to give
tonality to areas of the print) and flashing to give detail to
highlights.

Cheers
Gareth

Richard is absolutely right. I have verified it emirically. Split-grade
printing is a myth.

The crystals don't care whether in what order they are exposed. The
contrast obtainable  by simultaneous or successive exposure is exactly
the same.

You can get exactly the same effect by ONE exposure.

No, it's not. You may be seeing some difference in contrast because you
have not matched the two exposures properly.

Yes, it's an interesting issue.

I use filters.  With the type of negatives I use split printing for
(i.e badly exposed ones), I use long grade 5 exposures and very short
grade 0/00 exposures.

I suppose that, in strict theory, this translates into somewhere
between grade 4.5 and 5 (essentially, magenta with a splash of yellow).

But the splash of yellow is a very precise thing.

Theoretically, that could be reproduced with a single exposure with a
colour head - but you'd have to test to get the yellow right. So, you'd
probably start with a magenta equivalent to a grade 5 - get the
exposure right for the blacks - one test strip.   Then you would need
to start adding yellow until the highlights / mid tones were ok (one
test strip at least, perhaps more)

Meaning, I suspect you'd be in the same place - but with at least 2
tests, perhaps more.

Any thoughts?

Regards
Gareth