A traveller wants to join two prayers but he thinks it most likely that he will reach his city before the end of the time for the second one

So long as a person is travelling, he may join prayers, even
if he is going to arrive in his city before the time of the second
obligatory prayer begins, but in this case it is better not to join the
prayers, because joining prayers is only to be done when there is a need for
it. This man who knows that he will arrive before the time of the second
prayer begins does not need to join the prayers. Nevertheless, if he does
that, there is nothing wrong with it. If he arrives when the time for the
prayer has not yet begun, then he has discharged his obligation and he does
not have to pray, because he offered the prayer by joining it with the first
prayer. End quote. 

Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him)