Since Eric Prince creamated mr. Lerby and Chuck Houston together, why didnt he just creamate Chuck Houston with Verity Chandler in the first place?

Then Chuck's remains would have truly been disposed of over the Hollywood sign.

Interesting retrospective thought.
In the flow of events, the first order of business was disposing of Ms. Chandler, the murder victim.
The double-burning option better suited Lerby-Houston because Lerby, due to a boiler incident and being a 98-lb weakling, had a smaller amount of remains. Prince also intended to cremate Houston sooner, but the burner broke and the unit was down.
Anyway, the other mounting evidence against Prince was doing him in.