What's the feeling like of learning two languages at the same time? Is it super hard?

I don't think I can learn two languages at the same time, at least it should be different systems, maybe you can learn Japanese and Spanish at the same time, but not English and Spanish. (Japanese and Spanish are the best choices if you want to learn two langues at the same time as they have the same vowels and pronunciation)

Unlike learning English, Spanish learners don't follow Mandarin logic to learn Spanish, we are affected by English the most, and I think that's not an issue related to learners' English skill, just because these languages have same symbols.

The question should be changed to "What's the feeling like of using the second language to learn the third language?"

You might think that's strenuous, but it's not that hard if your second language is good enough. What's "good enough"? If you can watch unboxing videos in 1.25x of speed on Youtube in your second language and you're able to catch 90% of the speaking, it means your second language is good enough.

So, what the problem is?

It's just the same as learning the second language, you don't struggle on your native language and the aimed language anymore, instead, now is your second language fighting against the third language. As you've made such enormous effort on the second language, you may thought that you're losing your second language so that you're brain refuses to accept the third one.

One thing you can do, is to convince your brain that mixing languages up is common, don't be afraid to mess up in all of your languages.