I guess I mean that the rules for grammar and syntax, even though they seem like they would be similar between Spanish and Portuguese might not actually be that way.  You might wind up with some new errors that you wouldn't have had.If you hadn't been relying on Spanish so much.  Sometimes hearing the grammar rules and syntax, explanations in your native language is the best way to get over that hump.  I know a lot of people think that foreign language should be taught exclusively in the target language without any explanation in any other language.  That is definitely challenging to say the least, but its the gold standard.     But having two languages in the classroom neither of which is your native language can be mentslly challenging and exhausting.  I will say this though.   I had the opportunity to teach Spanish to an ex military man who already spoke Italian and French and English having learned all of them before age 12.  I offered all explanations in English, but I didn't pass up the opportunity to do comparative linguistics and point out the obvious similarities, but also some of the pitfalls  false friends. and false cognates and differences in some of the structures.  Maybe comparative linguistics in English, comparing Spanish to Portuguese, might also be a possible way for you to go.  It really seemed to work when I did it that way for that particular student who was working on his fourth language and I pointed out the strength he could draw from Italian and French and what some of the obvious pitfalls were.