Well I can't see any possible reason not to mix half as much stock, since you're going to dilute it further before use. Diluting the replenisher right before use MIGHT be different. Some developers change activity after mixing (like D76) but I doubt this applies to LPD. When I run out of the current liquid concentrate I may try that with my unopened powder when I mix it.
The concentrate bottle also says to write for info on replenishment. Do their instructions say anything about replenishing it? It really seems you should just be able to use more concentrate as long as you dilute it correctly, since that's all you're doing, albeit in two stages.