You want to have a separate config directory for your 2nd install.
One way is to start lazarus with --primary-config-path=d:\lazarus\trunk\conf
The easiest way is to create a file called
lazarus.cfg
in d:\lazarus\trunk (next to your new lazarus.exe)
and put the above line into that file.
You can then configure your new lazarus to use the existing fpc in the existing path.
Do not copy or move your fpc folder. If you do, you need to edit fpc.cfg
You can point to the existing gdb, or copy gdb, or download any gdb from our sourceforge page.
Or you can switch to fpdebug, and not use gdb at all.