For a cleanout routine it's a good idea to think about what you're doing. It seems pointless to take a laxative, then start enemas before the laxative has had time to work and has, at least, started to camp strongly. If you wash out the laxative med before it has started to work at least, there was no point in taking it.
As to a dose of MOM at night followed by an enema in the morning, that does make sense. MOM is a saline laxative, and those hold water in you bowel softening hard poo. Generally, the softening effect happens with lower doses of MOM. If a bigger dose is given, it can act more as a laxative because the increases bulk of the poo tends to be laxative.. In large doses, it can cause urgent, frequent, sloppy poos.
So, if your mom gave you a dose of MoM at bedtime, it may have been to soften the poo and make passing it after the enema easier.
MoM, as a preamble to an enema, does make sense. As mentioned above, it tends to soften poo, but also tends to move poo down in you bowel, so when the enema is given, even if it doesn't go very high, the person has a very complete poo.
I know my colon hydroterapist used to suggest a dose of MoM at bedtime the night before a colonic, even though every session stated with a bag enema as a prep to be pood out before the speculum went in. When I had sessions like that the single, 1500ml disposable bag, enema of plain warm water she gave me soon produced a massive, very easy to pass, toilet filling poo. The subsequent fills and releases on the colonic machine returned pretty clear every time.