Two separate ISO's, one each for x86 and x64 images is far less work.
1. Download all 4 ISO's, and 7-Zip extract to separate folders.
2. Load each of the W10 install images one at a time (if they're ESD format instead of WIM), and convert to WIM.
3. Unload the converted WIM images.
4. Select W7 x86 image, and right-menu -> Export -> WIM -> Save [as W10 x86's install.wim]. When asked, specify Append instead of Overwrite.
5. Select W7 x64 image, and export (append) to W10 x64's install.wim
6. Now work on each of the combined W10 + W7 install.wim's. You can share the same unattended.xml file for both releases.
7. Make the two separate ISO's, one for x86 and another for x64.
Because W7 and W10 are entirely different Windows, you must use a different preset for each version. But you can re-use the same preset for both W7's, and another preset for both W10's. You must use the W10 ISO's as the base, because W10 WinPE can install W10 or any older version.
Before you try modding the images, just run through the 7 steps to make sure you get bootable ISO that let's you install either Windows. It's better to know the process works, before blaming a broken install on these steps.
1. Download all 4 ISO's, and 7-Zip extract to separate folders.
2. Load each of the W10 install images one at a time (if they're ESD format instead of WIM), and convert to WIM.
3. Unload the converted WIM images.
4. Select W7 x86 image, and right-menu -> Export -> WIM -> Save [as W10 x86's install.wim]. When asked, specify Append instead of Overwrite.
5. Select W7 x64 image, and export (append) to W10 x64's install.wim
6. Now work on each of the combined W10 + W7 install.wim's. You can share the same unattended.xml file for both releases.
7. Make the two separate ISO's, one for x86 and another for x64.
Because W7 and W10 are entirely different Windows, you must use a different preset for each version. But you can re-use the same preset for both W7's, and another preset for both W10's. You must use the W10 ISO's as the base, because W10 WinPE can install W10 or any older version.
Before you try modding the images, just run through the 7 steps to make sure you get bootable ISO that let's you install either Windows. It's better to know the process works, before blaming a broken install on these steps.