Oui! I was actually just about to post this as well!
I noticed that there is no way to flag certain drivers (for example, Storage and Network), which are the only ones to be updated in Boot.Wim, and the balance of drivers (Video, Sound, Mgmt, Input...in addition to the original Storage and Network) that should be applied to Install.wim.
So for now, it must be done as a two-stage (or three-stage process).
1) Load boot.wim exclusively, integrate your storage and network drivers, apply save in WIM Format (optionally trim editions WinPE)
2) On the second pass, load install.wim -- load all your drivers ( NOTE: it is important NOT to flag "Update Drivers -> Boot.Wim" on the apply section.)
The tricky part is this:
Loading install.wim seems to still read the driver list from the corresponding boot.wim you just edited in Pass/Stage #1, so it gives you a warning about duplicate Storage/Network drivers if you try to load all drivers; of course, you're not planning to re-merge Stage #2 changes into Boot.wim, but you "cannot not-merge them" into Install.wim otherwise (I think?) The OS will not work on 1st boot .
Or will it?
(So here's the $1,000,000 question I'm trying to answer this week: Do Storage/Network drivers loaded from boot.wim automatically get loaded into the destination/target/resulting system even if they are absent from / not-merged into Install.wim?)
In a POSIX/UNIX environment, they would not.
For me this works out okay, because I actually need to run Stage #2 several times, for several different target languages.
But there is also a separate issue for which I will post a separate thread:
Excessive bloating of boot.wim if you run everything a single-pass process.