I spent several days watching the proceedings as they happened. Yes, formally there were two charges, but because the investigative report mentions other things those topics can be raised.
In attempting to resist impeachment, the Republicans want to call the report into question. That includes both it's direct evidence (attacking sources, resisting subpeonas, etc) and it's other claims. If you claim A and B and I debunk B, it sure feels like I weaken A (that's not logically correct, but people fall for this trick all the time).
RealClearPolitics, which is a Republican heavy source - so note here, they ARE biased - lays out the argument the Republicans used: https://www.realclearpolitics.com/articles/2020/01/28/democr...!
(and I'm not saying RCP is accurate as a whole, but I'm saying they are accurate as to the Republican argument against impeachment)