When i export as bibtex or .ris format and import into MyBib the authors are merged and therefore only one source.
I would assume this is something related to MyBib. You can open JabRef and look at the biblatex source tab or alternatively open your .bib or .ris files with a text editor and find that authors are not merged and rather be written in a separate way. At least for me it is so. Hence, I conclude, there is nothing wrong with JabRef’s export.
Rather something is wrong with the import into MyBib, because I tried to upload a .bib and a .ris file and it immediately triggered an error, aborts the import, but it does not say what exactly triggers the error.