Given how hard it is for anyone to get elected without the support of either the Labour or Conservative Party I think the main problem would be having them be the two front runners, as any constituency where both of them are likely to run is likely to face a major challenge from one of the main parties. The easiest way to get past this is to have them backed by the two main parties. This is possible for Galloway if you can prevent him leaving Labour (no Iraq War for instance), but getting the Tories to back Farage seems completley out of the question, given that he's the person they can point to and say "at least we aren't as bad as him".
One possible scenario might be for Galloway to run in a Labour/Tory marginal under a Labour ticket, whilst Farage takes the opportunity to boost his publicity by personally running against him as the UKIP candidate. The Tories position their candidate as the sensible middle-ground between the two, but a few days before the election their candidate dies or is implicated in a scandal so heinous as to utterly destroy their credibility (most likely involving paedophilia). Come election day, with a large section of the Tory vote either switching to UKIP or not voting, the result is a Galloway win followed closely by Nigel Farage.
But the case of Oldham West is probably not the best one.There was a former MP who was left wing who tried to win the nomination and did badly, but there was also another left wing councillor who was local who did a bit better, taking votes away from him. More importantly,Labour party rules mean that you cannot vote on MP nominations until you have been a member for 6 months. So the new membership intake did not get a vote.And then that leader would have to arrange it for this aging catherine wheel of shit to be parachuted in to a Labour seat in a by-election. That's not as easy as it sounds, because Labour candidates in by-elections are usually selected by the local Party, and in the only by-election we've had since Corbyn became Leader IOTL - Oldham West and Royton, which had one of the most left-leaning MPs in the Party until his unfortunate demise - the Corbynite candidate from out of town got zero votes by the local selectorate, who chose instead a local, Labour-right bloke. But even if the situation is such that the selection is not done by the CLP, that leaves it to the NEC - not the Leader - and it's hard to see how a Labour NEC would develop to prefer to parachute a lefty rebel like George Galloway over, say, Douglas Alexander.
So for Galloway to stand in a by-election for Labour, it would have to - at a bare minimum - happen after alt-Corbyn has packed the NEC with his own people the next time they're elected, which doesn't seem very likely.
But the case of Oldham West is probably not the best one.There was a former MP who was left wing who tried to win the nomination and did badly, but there was also another left wing councillor who was local who did a bit better, taking votes away from him. More importantly,Labour party rules mean that you cannot vote on MP nominations until you have been a member for 6 months. So the new membership intake did not get a vote.
So maybe if Galloway remained a Labour MP he could be nominated in the right constituency beyond 2016, but I agree the difficulty is finding a constituency were he could win and Farage also could.
Unless a seat becomes vacant before Ukip become an electoral force, and Galloway runs to win the seat, but Farage does it for the publicity, knowing he probably wont win. Maybe a scandal involving an ethnic minority community in that constituency could get him to do that?
In any case, he probably would not win, not only because of demographic factors, but also because of how a lot of the time before the rise of Ukip there vote in Labour areas was squeezed by the BNP, who were really like a working class Ukip at that point.