"thereby not only" is a grammatically correct phrase and can be used in written English.
It is typically used to introduce an action or consequence that is in addition to or connected to something previously mentioned. Example: "By choosing a plant-based diet, you can improve your health and thereby not only reduce your environmental impact, but also support ethical farming practices."
They are shunning them and flying out of regional airports instead, thereby not only sparing themselves a lot of aggravation but saving money to boot.
Bugs discovered in hardware during the software debugging phase require re-design and re-fabrication, thereby not only delaying the project but also increasing cost.
She thereby not only takes Mr. Bush off the hook for his disastrous decisions and policies, but also implies that her longstanding critiques of this president are merely predetermined by her genetic coding rather than arising from rational analysis.
1983), show that neither A.R.T. nor Lee herself could have obtained a copyright in the card-on-a-tile, thereby not only extending the period of protection for the images but also eliminating competition in one medium of display.
This will thereby not only contribute new tools to the field but also yield insights that can inform current theoretical conceptions of mental rotation and its relation to non-spatial processes.
By the time they close their front doors, the whole area will have been annexed to Jerusalem, thereby not only cutting the city off from a hoped-for Palestinian state, but slicing the West Bank in two.
Weaving Sacred Stories is thereby significant not only to the history of medieval art but also to art history and cultural studies in general.
She discovered thereby that not only had her husband had a mistress but that he had also murdered that mistress in their Sussex love nest.
It would thereby legitimize not only whatever the NSA may now be doing but lots of other surveillance it might dream up.
Far better to make pre- or post-cruise restaurant reservations, thereby avoiding not only the buffet but also the need for a table.
The German, who won the US PGA in 2010, thereby defied not only the draw but crazily long pre-tournament odds of 66-1 by scoring back-to-back nines of 33 – crazy, that is, in respect of Kaymer's ability.
