Yes, you can. That should be no problem. You’ve already got a decent level in English, so go for it with Mandarin! Continue to “work” your English by enjoying TV series, reading articles online, and so on. I suppose you could study Mandarin from English (rather than Portuguese) to get a good review of the basics. (Once you’ve got the basics under control, try to use more immersive methods.) The main issue is that Mandarin is going to take a lot of time for you to learn — probably four times as much time as it took you to reach the same level in English. Be prepared to enjoy a long journey.

Yes, I think so. Actually I expend almost all of my free time in learning English. Share it in two parts certainly will be challenger.

I'm learning Spanish and Japanese at the same time. Two similar languages would be difficult (for example, Spanish and Portuguese). However, two very different languages is much easier, especially if there's different writing systems. The biggest challenge is time management. You need to put in enough time each day for each language, or you won't go anywhere.