I see two way in which an original PPIE root could have split into
*dhe- and *do:- :
1) Because the roots are open syllables, a hypothetical following vowel,
similar to the thematic vowel, could have "entered" the root; this
is what I have proposed
http://tech.groups.yahoo.com/group/cybalist/message/46896
actually Piotr proposed something similar for the do:- thing
http://tech.groups.yahoo.com/group/cybalist/message/12130
and the now alternating root vowel would now one way or another
have influenced the inlaut consonant.
or
2) Hypothetically PIE might have had a system similar to the one in
Old Irish where verbs have an absolute and a conjunct form; one
for verb stems without preverbs and one for verb stems with.
Latin has something like that.
Torsten