dwh
Tail-End Charlie
The thing to consider is, to what extent is this detrimental? Is this really a concern?
Depends.
In your example, the FLA takes 18 hours to fully absorb, but the AGM is done in 3. So, if they are hooked together, the AGM will be held at absorb voltage (14.2 on your charger?) for 15 hours longer than it needs until the FLA is done simmering.
If it's cold out, it probably won't hurt it, but if it's hot out, then it might pop the blowoff valve and the AGM would lose water and the life would be shortened.