Nothing. It's traditional practice to present all the articles at once for efficiency's sake, but no requirement to be efficient.
In theory, a target could commit an offense, be impeached and acquitted, then commit a second offense. That would make for more than one impeachment.
Once you get creative, there is a near unlimited ways to abuse the process that haven't been "ruled out" by tradition or procedural norms. See what Pelosi is doing now. It is not expressly ruled out. Fortuneatly, there are norms that allow resisting the miscreant. At least in this case.
In Trump's case, the abuse of power in appointing SC and many of the other ongoing abuses have no immediate remedy. The process is stacked in favor of wrongdoers in the government. It's a good thing he fights. Usually the bad guys win.
thanx.