We are a small non-profit association with only a very small budget and have the following problem:

Our office is on the second floor and we have now rented two more rooms on the first floor of the neighboring building. Unfortunately, our internet connection as well as the WiFi router is on the right side of the building, whereas the neighboring building is on the left.

With one wAP ac as repeater at the end of our building, we just manage to provide one of the two rooms with WiFi. But since we have to supply the second room as well, the question is whether the following setup would be possible:

WiFi Router
(2.4GHz + 5GHz access point)
|
wAP ac 1
(2.4GHz client of WiFi Router)
(5GHZ point-to-point connection to wAP ac 2)
|
wAP ac 2
(5GHz point-to-point connection to wAP ac 1)
(2.4GHZ access point)

Preferably, the main WiFi router should act as a DHCP server for the entire network.

If so, it would be great if someone could explain us the necessary configuration steps in winbox for both wAP ac.

Thanks a lot!

There are quite a few factors to take into consideration, bandwidth, signal strength through the walls etc..
I would recommend setting wAP ac1 as the "main" AP and WiFi Router with the wAP ac2 are clients connected to it through 2.4Ghz (If signal strength is good, then 5Ghz)
All the firewall rules, DHCP-servers yada, yada.. will still be on the WiFi Router

I would stay stop wasting your time!
Simply get one of these two models
and create a wifi ethernet cable between the two buildings at near gig speed.
https://mikrotik.com/product/wireless_wire
https://mikrotik.com/product/wireless_wire_cube (has a 5ghz backup as well).

While your initial idea is possible, it will most likely cause issues: Your non-ROS AP will see all traffic coming from the same MAC address (wap ac #1 2.4 GHz). This is known to cause problems with some DHCP Servers, you should look for a different solution.

WiFi Router
(2.4GHz + 5GHz access point)
|
wAP ac 1
(2.4GHz client of WiFi Router)

This is the difficult part of the link. All Mikrotiks can make a wifi bridged network with no loss of functionality on that L2 network.
As the main router is not a MT, the first link must be via ethernet (or you have to setup a transparant L2 tunnel over wifi, what might be a challenge)
Once you are on a MT, all is easy, and all PTP links will work transparantly: WAP-WAP, SXTsq-SXTsq, Wireless cable on 60 GHz, or one of the other MT-MT links.

One of the steps might be adding a MT besides the main router (ethernet attached) for that wifi link to the first WAP ac1
The other solution is to add a "WIFI router brand" AP, ethernet attached to WAP ac 1 , and setup a WDS connection between Wifi router and that same brand device.
(Mixing WDS connections between brands is not supported).
Powerline Pro as this first link cable could be used, but may be slow or impossible.
All steps have only one purpose, make that "non-MT to MT" connection fully transparant on L2 level.

MT station-pseudobridge connection could work, but is only a "pseudo" solution, and as said by @whatever already: DHCP is one of the potential problems.