Depends on the type of deal and the circumstances of the election. If Brexit gets done before the next GE then I expect support for the Brexit party will have receded to a point where a deal would not be needed. If however, Brexit hasn't happened and parliament has blocked no-deal then I can see...
Farage wields influence (stops Boris backsliding on Brexit promises) and puts pressure on the Tories by not doing any deals but you are right the next GE will be crunch time for both men. Some sort of agreement will be probably be needed for the Tories to have a realistic chance of forming a...
Or Farage knows helping Boris retain a seat the Tories were expected to lose is likely to see even more Brexit party voters heading back to the Conservative party. By splitting the vote leading to a Tory loss Farage/ the Brexit party are showing they are still a big threat and could do serious...