OK.
There's no rationale behind it, but to each his own.
Just because you don't understand it doesn't mean there isn't one.
Here's a very basic RDI scenario for you, and this doesn't take much imagination given the plot provided by the note they themselves wrote:
I can't wait.
JR calls the pilot and cancels the flight (easy). JR gets in his car at 9:00 to go to the bank. He dumps the garrote, tape, remnants and wrappers from both, knife, wrist ligatures, sharpie pens, pineapple bowls, or anything else he KNEW was connected to the crime except for JBR (that being too conspicuous).
Well, that assumes he knew that much at the time (or at least was focused enough to remember it all).
He gets 118K from the bank.
Back home, PR pretends to wait for the phone call.
Okay, that's where I have to hit the brakes. I'm assuming that at this point, no one knows that there's anything wrong. By that I mean that they haven't called anyone over to the house, neighbors or otherwise. So just WHO are they supposed to be pretending for?
JR walks in at 10:00 or 10:30. Then they call 911 because they know JBR is in the house and they need her to be discovered and taken away.
There's a huge advantage for RDI to NOT call 911 at 6, and instead comply with the note while removing possibly incriminating evidence from the crime scene. This would further make it appear an intruder did it, by having more items missing from the crime scene.
I could buy that, under certain conditions. One, if these weren't two amateurs in over their heads. Two, if I thought that they both trusted each other. Just because they stuck with each other doesn't mean they were completely united.