I’m currently learning Portuguese through English on this website even though my native language is Spanish. By doing so, Can I keep studying English while learning Portuguese at once?
Si por supuesto. El portugues y el espanol son tan similares que solo hay que preocuparse con usar la palabra portuguesa corecta y no “una palabra espanol con accento portugues” a veces. Por ejemplo en portugues “rojo” es “vermelho.” Si estabas aprendiendo japones o algun idioma dificil, yo diria dedicar tiempo solo al nuevo idioma pero la verdad es que vas a aprender las estructuras y frases muy rapidamente en portugues y si has aprendido ingles por bastante tiempo antes, no vas a mezclar los dos idiomas.
[havent used spanish in years forgive any mistakes] 
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Same here, I’ve been using English to learn Japanese and Portuguese, I think I got less distracted by the similarities between Spanish and Portuguese that way.
Answering your question: I noticed that when I was focused on vocabulary (Japanese or Portuguese) sometimes my English vocab increased just a little whenever the native definition was too complicated and had to use a translation. As for “active” learning, nowadays I try to read a couple of books in English per year.
I am learning Spanish and could understand more or less understand 90% of that lol
wow! Where did you learn spanish?
By myself 
En espanol es “por si mismo”? Tambien habian muchos hispanohablantes en mi escuela secundaria.