EasyBCD is not a 3rd party boot manager.
It's an app to manage the contents of the Vista/7/8 BCD
It's still MS doing the booting.
You can add W7 to W8's BCD in exactly the same way as adding W8 to W7's using EasyBCD.
In fact, only the former will give you a workable dual-boot, not because of any problem with EasyBCD, but because the W7 boot manager does not recognize the digital signature of the W8 boot loader as genuine, so it will refuse to boot it.
When you add W8 to an existing W7, the W8 setup will take over the boot process and replace the W7 boot files with the newer versions, avoiding invalid signature problems.
If you add W7 to an existing W8, there's a danger that W7 might downgrade the boot files and hit the above-mentioned problem.
I can't be explicit that it will happen, because I've never tried it, but it is analogous to the situation with XP installed after Vista/7/8, which is guaranteed to regress the boot and make the newer system(s) unbootable until the boot files have been repaired.