> If that violent mob was successful in 'stopping the count' - then it very seriously threatens the legitimacy of Biden being president.
If it just stopped the count, and Congress took no other relevant action thereafter, the Presidency and Vice Presidency would become vacant at noon on the 20th and Nancy Pelosi, by virtue of her office as Speaker of the House, would succeed to the Presidency.
But supposed the Congress (or a sufficient subset to deny a quorum in either House) were detained, and Grassley and Pelosi were killed. Then, when Trump's term expired, Trump loyalist Mike Pompeo (while cabinet members traditionally submit their resignation at a change in administration, they don't automatically leave office) would be next in line of succession with vacancies in the Speaker and President Pro Tem positions.
And, more likely the goal, what if Pence were killed (and probably Grassley, too) but then.Senators were released and a President Pro Tem amenable to exercising the arbitrary power Gohmert suggested the presiding officer would have in the electoral vote count were elected, and presided over the vote count as the Vice President was vacant?
(I do think any scenario results in “two Presidents”, I do think several result in “one President (in some cases of dubious legitimacy) who isn't Joe Biden, and (in several scenarios) is either Trump or a Trump loyalist."