Is it necessary to allocate multiple channels to an access point at the same time?
In short answer that is correct, if a network is allocated multiple channels then it needs all those channels at the same time to operate. We cannot schedule the network in different time window.
You are using the basic concept of OFDM where in you divide your channel into multiple frequency and operate you're communication system in those channels, Note that when you divide your channel in frequencies you're basically extending the amount of time to transmit the same amount of symbols,hence while using N4 AP it is better for performance reasons to alot both the channels at the same time otherwise you would have a worse performance.
Also in your scenario i would suggest not dividing the channels at all you could use CDMA for the N1,N2,N3 for T1 and use N4 at T2 this way you'll use the maximum spectral efficiency and you're bandwidth wont be idle.
Which is a combination of TDMA and CDMA(Time Division Multiple access)