Velenos didn’t say the result would be stronger.
This seems like doing twice the work for the same result.
Pre-ritual work is similar to what you’re referring to, if I understand you correctly. Regardless of whether you petition or do pre-ritual work, they should be expecting your call.
And, as Velenos said, it’s their choice to come or not. And if they do come, you have to work out a way to communicate. Even if you evoke them to physical manifestation (which isn’t necessarily needed for results), you still have to have a way to communicate. If you’re clairs are developed, you’ll want to have another method ready, such as tarot or pendulum.