Results are now being counted after ballot boxes closed at 10pm (GMT). The ballots are the result of two mid-term Labour resignations, with Jamie Reed quitting as Copeland MP to re-join the Sellafield nuclear decommissioning plant and Tristram Hunt leaving the House of Commons to become the director of London's Victorian and Albert Museum.
Both were Corbyn critics and both have left the Labour leader open on two fronts.
In Copeland, the Conservatives are hoping to overcome Reed's majority of more than 2,500 votes from the 2015 general election, capitalise on Corbyn's anti-nuclear credentials and defeat Labour Councillor Gillian Troughton.
In Stoke, Ukip have targeted some of the 69% of the city's residents who backed a Brexit at the EU referendum.
They have rolled out their second biggest name, Paul Nuttall, Nigel Farage's successor as Ukip leader, to fight for the Staffordshire seat.
But the Bootle-born politician's campaign lost momentum after he admitted he did not lose "close personal friends" at the 1989 Hillsborough football stadium disaster, contrary to claims on his website.
The vote will also determine whether Ukip have a place in British politics after the Leave vote at the EU referendum in June. "Even by the time of by-election I would only be three months into my leadership, give me time," Nuttall told IBTimes UK on the campaign trail.
"But I'm not contemplating losing, I think we're in with a really good shout here, there's a buzz around us, there's a buzz around the place, probably that Labour don't have at the moment."
Labour have selected Gareth Snell to defend Hunt's majority of more than 5,000 votes. The local councillor described the Brexit vote as "pile of s**t" and called Corbyn a "friend of Hamas".
But despite his controversial tweets, which he's apologised for, the odds are in Snell's favour. In fact, Conservative candidate Jack Brereton, 25, could have a good showing as the Tories ride high in the opinion polls and after Theresa May visited the seat.
On top of that, here is a statistic, via Number Cruncher Politics founder Matt Singh, to focus the minds of Westminster watchers when it comes to Copeland: no governing party has gained an opposition seat with more than a 3% majority in any by-election since Worcester in 1878.
In other words, Labour's immediate future would look bleak if the party loses the West Cumbrian constituency to the Conservatives.