Legally you can't use your Win7 installation in parallel with your Win10 installation. Technically though, you can.
You are allowed to revert your Win10 installation to Win7 (or Win8.1 if your upgraded from there), within the first 30 days after the upgrade. And you can again upgrade to Win10 later if you so desire, even after the free upgrade period has passed, as the license has already been registered at MS to be eligible for upgrade.
I haven't tried to do a fresh install with a Win7 license that was already upgraded to Win10, (and I don't intend to, ATM) but you can ask google what would/could happen then... I suspect the same legal issue as mentioned above.