I started Arabic and Spanish simultaneously and didn't have any issues. As long as you organise your time and accept that your rate of progression will not be as rapid as if you were studying a single language, I think it's perfectly doable.

I agree with others who say it is possible to learn more than one language simultaneously if (1) the languages aren't too similar (like Spanish and French) and/or (2) you start the languages at different times and therefore, have a different skill level in each language. Right now I'm studying Chinese and Arabic. They're SO different, I never confuse them. (Sometimes I remember a word in one language and forget it in the other, but I never mistake a Chinese word for an Arabic word.) Also, I have a higher level in Chinese so my abilities and goals are different than my low level abilities and goals in Arabic.

Also I agree with others who say it helps if you can organize your study time in a way that works for you. I spend 10-15 minutes every day reviewing flashcards in both languages, but I don't like to learn new material in both languages on the same days. I like to split my week so I have Chinese lessons or chats on Sat-Mon and Arabic lessons or chats on Tues-Thurs.

I think you can but you shouldn't start at the same time. Once you have reached the intermediate level you can start with a new language.

I've been learning five languages at once. It's really hard. I dedicate a week for each languages I learn, so in a month I go through 4 languages. It works for me so far though progressing slowly.