Bootloader on USB Flash Drive


I have a work laptop with a single 1TB SSD that currently has a business license of Windows 7 on it. I've created a second partition on the drive to install Windows 10 (a personal copy). I'd like to have a bootloader on a USB flash drive so that I can leave the setup exactly as is and when the USB drive is inserted it gives the option of booting to either W7 or W10. When the flash drive is removed, the existing bootloader on the SSD will automatically boot into W7 as it currently does. In other words, I don't want to overwrite the existing W7 bootloader with a new one with multiple OS entries. Is this possible?