An article talking about confusion with conditionals and the phrasal verb to put off.
My other question is connected with the phrasal verb to put off. Which preposition we use after this phrasal verb in the meaning of to postpone? e.g. I want to put this meeting off (for?) two weeks.
Answer to question 1 – conditionals
First of all, Agnieszka, you're quite right that in conditionals you can use any combination of tenses that makes sense or expresses what you want to say. The sentences you're asking about aren't strictly speaking conditional: 'making up their minds' is of course dependent on 'looking round the house', but there isn't any 'if', any doubt about whether they're going to look round the house - they're going to look round it and therefore they're going to make up their minds.
The only difference between your sentences a) and b) is that sentence a) emphasises that they'll finish looking round before they decide - i.e. there's a definite separation in time, however short, between the two events - whereas sentence b) doesn't have this emphasis, and suggests that the two events happen simultaneously, and that perhaps they'll decide before they've really finished looking round. But in practical terms this difference is tiny.
However, there's perhaps a bit more to be said about the use of as soon as. It's normal to use as soon as to mark the passing of a point in time, e.g. the point at which they finish looking round the house, so this is fine:
They'll make up their minds as soon as they've looked round the house tomorrow.
But it's less usual to use 'as soon as' for whole events with an obvious duration, such as looking round a house: They'll make up their minds as soon as they look round the house tomorrow.
An better alternative would be:
They'll make up their minds when they look round the house tomorrow.
We also use 'when' to mark the passing of a moment, so this is also OK:
They'll make up their minds when they've looked round the house tomorrow.
Answer to question 2 – phrasal verb 'to put off'
Yes, I want to put this meeting off for two weeks is fine. The phrasal verb is put off, and for is part of the phrase for two weeks. So this sentence is equivalent to I want to postpone the meeting for two weeks. Similarly, you could ask either: How long do you want to postpone it for? or How long do you want to put it off for?