The problem with trying to improve two languages simultaneously is code-switching. Code-switching occurs when you switch from one language to another. It is relatively easy for me to switch from English to Polish--unless I have been furiously studying Italian for two hours.
This was the sad situation I was in on Sunday when a parishioner introduced me to his girlfriend, visiting from Poland, and I said "Miło mi poznać" ("Pleased to meet you") and then totally blanked out. I even forgot my "magic word" for such occasions, which is "Spokojnie" ("Be calm").
Today I studied Italian all the way to exercise class, but then listened to Polish lessons on my phone all the way home because my Polish-speaking help was coming, and I didn't want another brain-freeze. Alas, the help messaged to say she is sick today, so I might have spent another bus ride working on Italian, but there it is.
One of my sisters is fluent in both French and Spanish and is now learning Italian, so I must find out how she manages. I suspect she has an unfair genetic anomaly, though. Possibly the left side of her brain talks to the right lickety-split.