Can both clients be installed and used at the same time ?
The purpose of using both is based needs to develop for Lync 2010 while still using Communicator 2007 in a corporate environment. Or would a developer be forced to have two different machines/VMs/boot environments ?
No - you can't run both side-by-side. Installing Lync will uninstall Communicator.
However, you can use a registry hack (unsupported, unfortunately) to enable your copy of Lync to run against your OCS infrastructure. This works for R2, not 100% sure about R1.
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Communicator]"DisableServerCheck"=dword:00000001
If you're serious about Lync development, the best solution is to build a server environment specifically for developing against.