When dual-booting, the situation is that you're only using one license at a time--so if you boot into Win7 you aren't running Win10; if you boot into Win10 you aren't running Win7, etc. I see that as similar to the retail licensing for Windows 7/8--buy the retail license you get the 32-bit & 64-bit versions, both, but only one at a time is licensed for use. Both versions are licensed, but may not be run concurrently. Not really sure how the authorization servers might deal with that, though...
At any rate, unless you're having some kind of Win10 driver-related problems, I see no reason it cannot immediately become your primary OS, rendering dual-booting just an unnecessary headache with no advantages that I can think of. Even if you start out dual-booting you'll soon find yourself spending all of your time on Win10, anyway--and then it's just a simple matter of bypassing the bootmenu and booting straight into Win10 and making use of that partition disk space...A good use for it is as a backup for your Win10 partition...That's what I did when I decided Win10 was stable enough, a few builds back...
I did forgot about something when doing a triple boot, updating each operating system takes time. I just hope windows 10 doesn't see the windows 7 retail as a duplicate version. I guess when I exit the insider program I will find out.
You cannot have the same license installed to more than place, except as part of a backup. And you can't call dual booting a backup.
You do have a note of your original OEM key don't you
However I would stay with the insider program for a while until any issues that may occur with upgrading oem systems have been addressed. There is various conflicting statements flying around. I also suspect that you also have the windows 10 upgrade icon on your retail version. The upgrade to retail 10 may decide to use that as a basis so I would clone/image that partition.
Do not rush the upgrade see what gets posted about any issues once the retail 10 is available.
I don't want to start a war here, and I'm not pointing fingers either, but I have some problems between technically possible and legally possible.
Microsoft has put $Millions$ into Windows 10 and have allowed us to use it free (warts and all) for almost a year. Further, they have offered a free upgrade to mostly consumers (mostly being a qualifier) from genuine Windows 7 and Windows 8.1.
I am satisfied with that offer and think Microsoft has been very generous with us. I have genuine Windows 8.1 on my four active computers. I have seven genuine licenses to Windows 7 that I am free to use on any computer. I wish I could just give these keys to those who deserve it, but they're nontransferable keys, so I can't.
If one has a computer with Windows 7/8.1 on it and simply upgrades to Windows 10 on July 29, that is legally possible and is in the spirit of Microsoft's offer of the free upgrade and EULA.
If one has a computer with Windows 7/8.1 on it and bypasses Microsoft's EULA (no matter how it's accomplished) in order to use both the original OS and the upgrade to Windows 10, that is technically possible but is not in the spirit of Microsoft's offer of a free upgrade. In fact, there are some, including Microsoft, who would call it downright piracy.
That is why I made the distinction.... I didn't want to imply that it's impossible to do something, because it's not. However, if you want to be legal, then you cannot install the same license on different partitions (and since the Free Windows 10 upgrades your old license, it's still considered the same license, even if the terms are different).
Even if you have multiple keys, that's not multiple licenses.