I had been doing that as well, as that worked with adderall and caffeine for me, but I noticed if I took more later in the day, my quality of sleep diminished.
I came across some info that demonstrated duration was largely determined by dose. The prescribing guidelines recommend larger doses, not split doses for extended duration.
The reason is Modafinil / Armodafinil duration is determined by rate of clearance, which there's a limited capacity for. (as a side note, the elderly need lower and lower doses for the same effect and duration because they clear it increasingly slowly).
So I just started taking larger doses first thing in the morning, and sure enough, while the peak was a bit higher, after leveling off the effects lasted significantly longer and my sleep quality returned.