You can have Botox injected at the same time as fillers, or fillers can be injected two weeks later. I prefer the latter method. It gives me a chance to tweak the Botox injections if need be. Injecting the two roughly at the same time, helps prolong the action of the filler since the Botox will spare you from at least three months of muscular movement. It is this muscular movement that helps lead to the dissipation of the filler.
The only rule is that the filler injections be done first. The fluid of the Botox can make the lines meant for correction less apparent.