In the book The Great Northern War and Estonia : the trials of Dorpat 1700-1708 (2010) the Estonian historian and diplomat Margus Laidre discusses the effort to relieve Dorpat in the summer of 1704. According to Laidre Major General Schlippenbach on 14 July 1704 wrote to Charles XII, suggesting that the King should order Major General Lewenhaupt in Courland to march to Livonia and join Schlippenbach in an effort to relieve Dorpat. Laidre notes that this was very late and that it would have taken a couple of months (at the very least) for a reply to arrive, which in Laidre's opinion suggests that Schlippenbach simply wanted to make sure that he wasn't blamed if Dorpat fell.
Laidre also writes that Schlippenbach enclosed a copy of a letter from Skytte, dated 4 July, which has not been found. In a footnote Laidre gives the volume M 1394 in "Riksarkivet" as the place where Schlippenbach's letter is preserved. M 1394 belongs to Schlippenbach's "field archive" and contains drafts of his letters from June-July 1704. To find the letter that was actually sent (and any attachments to it) one needs to look elsewhere, in this case in the collection "Skrivelser till Konungen. Karl XII" - "Letters to the King. Charles XII". There are three volumes of Schlippenbach letters - 23, 24 and 46. The latter volume contains the letters from 1703-04. And there the letter in question can be found - with the Skytte report attached. The advantage with using this volume is that the date when the King received the letters are often noted. In this case he got it on 2 September 1704 at Lemberg - much too late.
However, this was not the first time that Schlippenbach suggested that he and Lewenhaupt join forces. He had taken a similar initiative already on 9 June in a letter to Governor General Frölich in Riga. Laidre is also incorrect in saying that Lewenhaupt could not act without the express order from the King. Lewenhaupt had full control of his forces and could act as he saw fit - but Schlippenbach could not force him to do anything. It is also clear that Lewenhaupt, in total agreement with the King's intentions, for political reasons considered Courland and Lithuania more important than Livonia. By moving northwards he would abandon those in Lithuania who had joined the Warsaw Confederacy and give the Oginski and Wisniowiecki forces a free hand. Charles XII (and Lewenhaupt) were for the time being fully prepared to make sacrifices on the Russian front in order to reach the goal in Poland, i.e. the removal of Augustus.
Riksarkivet, Skrivelser till Konungen. Vol. 46.
Uddgren, H., Karolinen Adam Ludvig Lewenhaupt. I. -Stockholm,1919