To get dual boot with 7 and 10 you set your machine up and boot/install 7, then boot/install 10.
That gets me a dual boot menu. But this would require us to ignore that free Windows 10 offer and buy a license for 10.
While we wait to hear more about your method and what you bought for your licenses, look at EasyBCD.
How to make dual boot on separate drives? I have already installed Win10 on SSD and Win7 on HDD. I tried with Startup and recovery -> Time to display list of operating systems but I only got one option to choose (win10). I thought it would be easier with two different drives