Similar variations of this question have been asked a few times here.
In order to "merge" the contents of 2 servers sharing a single domain name you need to use a reverse proxy. This would mean that all content on server2 will go through server1 (or Vice-Versa) or alternatively you could have a stand alone reverse proxy divvying requests between server 1 and server 2.
This can be achieved using mod_proxy if you are using Apache. NGINX has similar functionality.
There is no mechanism in common use which allows you to split directories between 2 servers without some kind of reverse proxy or load balancing in common use. (ie conceptually for the purposes of HTTP/HTTPS a domain resolves to a single IP address = a single Interface on a machine)
https://folder1.example.com/
would be a lot easier to make work. Are you open to that?