Xylosyltransferase specific to UDP-D-xylose that accepts both cellopentaose and cellohexaose as substrates, with a better use of cellohexaose, to produce xyloglucan. Adds preferentially the first xylosyl residue to the fourth glucosyl residue from the reducing end of both acceptors. Transfer one xylose mainly to the second glucose residue from the non-reducing end. The acceptor should have a minimum of four glucose residues. Transfers an alpha-D-xylosyl residue from UDP-D-xylose to a glucose residue in xyloglucan, forming an alpha-(1->6)-D-xylosyl-D-glucose linkage. Binds 1 Mn(2+) ion per subunit. Protein modification; protein glycosylation. Forms homodimer (PubMed:29784804). Interacts with XXT2 (PubMed:22665445). Mainly located in the cis and medial cisternae of Golgi apparatus. Reduced xyloglucan content. Belongs to the glycosyltransferase 34 family.