I also take my long acting at two points in the day but I don't "split" it, I take an extra dose in the morning - I'll explain how I came to this. This worked for me, we may all respond differently.
I found 28 units of insulatard a couple of hours before bed (10/11pm) worked perfectly from day 1 (about 15 years ago). About 7 years ago I noticed my daytime glucose levels were less stable despite taking the right amount of insulin per carbs etc. I discussed this with my doctor and we both came to the idea of trying the long acting in the morning as well as at night. My Doc suggested splitting the dose, and trying 20 at night and 8 in the morning. That didn't make much sense to me as I was aware that the effect of the insulatard wore off towards the end of the 24 hour period, so the night time 20 units wouldn't be fully supported by the daytime 8 as that 8 dose would be declining through the night.
I decided to keep to the 28 units before bed (10/11pm), and very cautiously tried 4 units in the morning (8/9am). All good, blood glucose felt a bit more stable, though I felt there was room for improvement so worked up to 8 units in the morning, and that worked a treat. More than that was too much.
The theory
From the graphs I've seen showing the activity of long acting insulin, they seem to rise fairly evenly, peaking around 12 hours, then dropping off fairly evenly from there. So, my extra morning dose is slightly leveling out the uneven graph. I've never looked into it beyond that simplistic way, but will do. Maybe there is a better way to do this?