I started learning Mandarin for the same reasons you are contemplating Steve. I think it is a much better strategy to learn a completley unrelated language. Spanish is a Latin language and so are Portuguese, French, Italian and Romanian so they are all very closely related. I'm not familiar with Russian nor Polish, so I'm afraid I can't help with any comment on those. My guess would be that they wouldn't interfere, but maybe a new conversation directed at people who have studied Spanish and Polish or Russian that the same time might be useful to you. 

Mandarin has actually helped me with my Spanish in the way you were hoping that Portuguese would for you. The further away the language is from Spanish, the more different the sound and culture, I think the better. There's no way I could EVER get anything at all mixed up between Spanish and Mandarin. :) 

Best of luck to you with your new language! 

I would suggest something from a different language family than Romance. So Polish or Russian sounds good. You only want to delve into studying similar languages once you have a really good level in one of them.

Well thanks. I did an online proficiency test and it had me as an advanced beginner. I think my grammar is a little off as the last 6 months or so I have been concentrating on speaking. Comprehension is my biggest obstacle, I can speak quite well and often read quite well but when listening to speech I struggle. 

I have really began to like learning languages and wanted a secondary one to learn when I feel a bit overloaded with spanish and thought Portuguese would be ideal as it would compliment my Spanish even if I feel I'm taking a break from it. But maybe not. 

Maybe be I could drop the Portuguese for a bit and do something totally different like Polish or Russian as I'm assuming French or Italian would be too similar also. 

Don't do it unless you have at least a B2 or a C1 level in one of them because even for people with high levels in one of these languages, sometimes there is still confusion when learning the other.

Would you consider yourself advanced intermediate or above in your Spanish yet Steve? If you are still a beginner, I would pick one or the other because I'm not sure if there any two languages more alike than Spanish and Portuguese! When I was in Spain and feeling pretty set up in my Spanish I went for a visit to Portugal and could understand almost everything that was said to me, the languages are that close. Learning those two languages as a beginner I would think could be quite confusing. I just started learning a little Portuguese to see how it would work at my level and it seems ok because at this point I'm never going to get any Portugues into my Spanish, it will always be adding a little Spanish to my Portuguese. Spanish is clearly the foundation language even though I struggle with it. Still, I've decided to wait until next year in order to give myself an even firmer base in Spanish to start the Portuguese. Once I have a full adult vocabulary (both active and passive) in Spanish, then I will feel like concentrating on Portuguese will make sense for me.