While it may technically be possible to install more than one execution server to one host, it is not an officially supported best practice.
Running two execution servers on one host can cause stability and/or routing issues, especially since execution servers are required to have different IP addresses. If two execution servers were installed to one host, they would consequently share the same IP address.
Having two execution servers competing for system resources from the same host can likewise threaten stability and performance.
If one needs to run two execution servers on the same hardware, however, an alternative would be to dedicate virtual hosts for each execution server, in which case each virtual machine would have its own IP address.