As an example, take the case of someone who speaks English as an L1, and wants to become fluent in both Spanish and French. If they learn Spanish first and French second, will they tend to be more fluent in Spanish than French (assuming the same time was spent studying each language)?
Keep in mind, the above is only one instance and by no means am I restricting this question to solely French and Spanish. For any two languages and as an overall trend (not on an individual, case-by-case basis), do you tend to be more fluent in the first language you learn rather than the second language you learn?