After I use the masstagger feature to auto track number, it would be good to be able to sort renamed tracks in Windows explorer. Now if I auto track number and sort on name, I get something like this:

Red Hot Chili Peppers-1-Under the Bridge.mp3
Red Hot Chili Peppers-10-Breaking the Girl.mp3
Red Hot Chili Peppers-9-Parallel Universe.mp3

If mastagger used two digits in auto track number, it would sort by name like this:

Red Hot Chili Peppers-08-by the Way.mp3
Red Hot Chili Peppers-09-Parallel Universe.mp3
Red Hot Chili Peppers-10-Breaking the Girl.mp3

It's just a small detail, but it makes auto track number much more useful.

$num(%tracknumber%,2)

(Reading the documentation helps)

Please use the search function. This has been discussed like a zillion times.

I searched carefully before asking, and found nothing.

To accomplish what you ask, make a new masstagger script containing the following two actions:
1) Auto track number
2) Format value from other fields...

The latter configured like this:
Destination field name: TRACKNUMBER
Formatting pattern: $tracknumber()

Note: The following fomatting pattern will also work: $num(%tracknumber%,2)

Save this script as e.g.: Auto tracknumber XX
and you'll have easy access to it whenever you need it, by pressing Add or Load after locating it in the drop down list.

You can keep the single-digit tracknumbers, and use $tracknumber() in your renaming string.

Perfect. Thank you very much.

No problem

Btw: If you add kjoonlee's solution to your renaming scheme, you won't need to use a new script, or need to retag your old single-digit-tracknumbers (if any). Using both won't hurt either...

IHMO "the kjoonlee way" is better and also the one I use myself.