It is most helpful to compare the three accounts of this question by the disciples in Matt 24:3, Mark 13:4 and Luke 21:7. All consistently say that it was a private meeting between the disciples and Jesus and all record that the first question was, "Tell us, when these things will be". It is the next part of the question that varies between the accounts.
- Matt 24:3 - and what the sign of your coming and of end [συντελείας] of the age
- Mark 13:4 - and what the sign when all these are about to end [συντελεῖσθαι]
- Luke 21:7 - and what the sign when these things are-about to-take-place [γίνεσθαι]
If we assume (as I must) that these independent reports are all accurate reports of the sense of the conversation (presumably in Aramaic?), then it immediately follows that the Greek wording we have are, in there essence, equivalent in all three cases.
It should also be recalled that this question was a direct response to Jesus' "teaser" statement following the disciples' grand tour of the magnificent temple. Then Jesus calmly remarked that the temple would be destroyed and all its stones thrown down. It is immediately clear that the disciples interpreted their temple's destruction as the coming of Jesus and the end of the age. (This was a constant problem that Jesus worked hard to dislodge.)
Therefore, it is not surprising that the disciples posed such a question so as to be prepared. They wanted warning - specifically a sign/omen - that this significant event was about to happen.
Note that the "sign/omen" of the event is couched in different terminology and different grammar and parts of speech so we should not press the precise words to hard other than to suggest that the three phrases are somehow equivalent:
- your coming and of end [συντελείας] of the age
- when all these are-about to end [συντελεῖσθαι]
- when these things are-about to-take-place [γίνεσθαι]
The future verb to be "ἔσται" [= will be] in the first part of the question must apply to both parts of the question, despite the verbs in two cases having present tense: συντελεῖσθαι, γίνεσθαι. Even without the future verb to be, the second part of the question in all cases is still conspicuously future oriented.
Thus, the disciples question can be condensed to something like, "when this will-be?, and what sign of the coming calamity?" Note in this condensed form I have had to use both a future tense (will be) and a present tense (coming).
In His response, Jesus was very gentle and made no attempt to rebuke the disciples that the destruction of their all-important temple would automatically mean the end of the age/world and the Jesus' second advent. Instead, He appears to combine the two events, the destruction of the temple and His second coming into a single prophecy and use the former to help explain the latter.