Hi Peter

I can't see how this is going to be possible using ordinary outline numbering. The reason is that you would need to create a
circular argument: the numbering for a Heading 2 paragraph depends on the preceding Heading 1; but the numbering for a Heading 1
paragraph depends on the preceding Heading 2. Computers aren't good at circular logic like that.

The proper way to solve this problem is to use SEQ fields. There's info about SEQ fields in Word's help.

But what most people do is cheat. You can exploit a flaw in Word's object model and proceed as follows.

Ignore, for the moment, your need for Heading 2. Set up your numbering as described at
http://www.ShaunaKelly.com/word/numbering/OutlineNumbering.html as follows:

1 Heading 1
2 Heading 1
   a Heading 2
   b Heading 2
3 Heading 1
   a Heading 2

Now, put your cursor in a Heading 1 paragraph. Format  > Style > New (or Format > Styles and Formatting and click New Style). Give
your new style a name like Heading 1_X. Note that this new style is based on Heading 1. And owing to a flaw in the object model, it
will take on Heading 1's numbering. So you can have something like the following:

8.  Heading 1 or Heading 1_X
   a. Heading 2
   b. Heading 2
   c. Heading 2
9. Heading 1 or Heading 1_X
10. Heading 1 or Heading 1_X
   a. Heading 2
   b. Heading 2
   c. Heading 2
11. Heading 1 or Heading 1_X

Note that you will have problems in copying paragraphs numbered in this way from document to document (the exploitation of the flaw
doesn't survive the copying). And don't count on this lasting forever: Microsoft might one day figure out that you can do what you
shouldn't be able to do!

Hope this helps.

Shauna Kelly.  Microsoft MVP.
http://www.shaunakelly.com/word
Melbourne, Australia

I guess the first question is why you want to do this.  It seems to be a
fundamental problem to mix the levels in this way and maybe there is an
underlying reason for your choice.  If you want the formatting to be
different for the two levels but to have functional and appearance
similarity in all other ways then maybe there is another way.  

For example it is common in published papers to have similar headings
for Abstract, Introduction, Methods, Results, Conclusions, and
References, yet only have the middle four with numbers.  To do this I
would make all of them say Heading 1 but just click off the numbers in
the first and last (by clicking the number box in the ruler to the off
state).  

So if you can provide a little more on the reasons for this choice and
we may be able to see get arounds.