Evandro, this sounds more as a discussion than a specific language question maybe. You might get more reaction in the discussion section on this.
For giving my two cents here then:
1. I think that giving any subjects, if you study one thing, you will make more progress than when you are studying two. Languages is no special case. Of course you would learn English faster if you only focus at English. But the same goes if you would for example study mathematics and want to take up chemistry as a second subject.
2. Some people say that you could get confused between the two languages. This can, but I think the confusion is not thát troublesome. In my opinion you can easily study more languages at the same time. The only thing is that languages that are somewhat related should be more or less on the same level.
For example, I am at 'C1-B2' in English, German and Spanish. I also speak French. Since my English and German are more or less at the same high level, I hardly ever mix them up. When I try to speak French though, I put in Spanish vocabulary. But that is just because I know Spanish much better than French.
In that light, I would say, start studying German as quickly as possible to lift it up to your English level. After that, study the both languages together.