Yes it is possible you can have ISPConfig pick the webserver... but there are 2 problems you will be faced with. One is the FTP, the the other is having the DNS records point to the right web server.
Now the second can be solved easily by using a ReverseProxy (in which if you router is forwarding port 80 to the first webserver, then you add information of domains located on the second webserver using the ReverseProxy feature).
So the idea is you use the first webserver as a base and domains located on the second webserver are being proxied through the first.
As for the ftp, I would be unable to tell you how to split that kind of traffic, I am sure there is a way (maybe even use the same method by defining records in the DNS then defining proxy of those records to the right server)