Is it possible to Install two (2) Firebird instances of different names & ports (gds_db port 3050 & fbsvc port 3051) running as service?
Yes,  it is possible and it is covered in documentation and in lot of FAQs like the one at iBase.ru 

For the example at my development box I have co-installed FB 2.1.5 Win32 SuperServer (at default port 3050/tcp) and FB 2.5.2 Win64 Super-Classic (at a custom 3064/tcp)

There might be troubles with FB 1.x as it was using registry, but FB 2.x was made isolated and self-dependent. 

One option is to download ZIPs and unpack them to different folders.  Then you have to run text window of Windows Command Prompt "As Administrator",  go into "bin"  subfolder and there are all those executables like server itself.  There also is "inst_svc" tool.  Running it with an option like "-?" would show you brief help. Focus on installing main service,  not installing Guardian (only needed on Win98, only shipped for legacy uniformity)  and giving non-default "instance name". Then go outside "bin"  and open "firebird.Conf" with any text editor like notepad. The documentation is within that file how to set non-default TCP port. That's all. Do the same for your another FB folder and you've done it. 

Another option is to run two installers.  The 1st one would do all the described above things automatically. The second one would unpack files -  and ask you to do those configuring operations for the second copy manually. Just do it like described above.