I learned French in high school and not Spanish (thanks to growing up in southern Louisiana), and I can't read Spanish or Italian texts unless I'm extraordinarily lucky about vocabulary, but I can read Interlingua just fine. I suspect I'd be hopelessly lost in an ad-hoc mashup of Spanish and Italian, but Interlingua is pretty good about constraining vocabulary to things that are usually obvious to a speaker of any Romance language and not adopting shifts that have only taken place in one / a few languages.