In ITOBATTR, I could not find any SHTXT field, Assuming that you are talking about TXT_EQUNR (40), this field and TXT50 field of ANLA are of different data types. If you wish to use such 2 fields for maintaining the text of same object, you may be required to use only 40 characters of the TXT50 field as a solution.
OR
You may use the longtext option of this text field (40 character field)
Jogeswara Rao K