Ughitssphie
Firstly I’d say ignore the naysayers and just go for it. If you’re motivated and dedicated enough it’s certainly doable.
In my own experience. I speak Japanese fairly well (after 4 years living in the sticks in Japan) and now live in HK and am trying to learn chinese (both Cantonese and Mandarin). I know I know I’ve thrown an extra spanner into the works lol.
Since leaving Japan i’ve tended to focus more on reading than speaking (for reasons of access mostly). I undoubtedly get confused sometimes. Occasionally pulling up a chinese word (when I was looking for a Japanese one) but hey I did that with Japanese and English especially when I’d come back home to UK and speak to English friends and blurt out Japanese then realise my mistake from the blank looks on their faces 
But those moments are few and far between and usually after a minute or so of reading/listenings/speaking or whatever you tend to slip comfortably into the language you are trying to use.
I’d echo what Jprspereira and others said about one tending to be stronger than the other. I certainly tend to have periods where i’m more focussed on one than the other. Usually it’s Japanese because my ability in that is higher but I certainly have times when I feel ‘burnt out’ from one and focus on another allows me to reenergize and focus.
With chinese I’m mostly definitely focussing more on reading than speaking. I find the tones incredibly difficult to nail down. (Especially Cantonese with it’s arguably 9 tones). But I really enjoy the character study whether it’s chinese hanzi or japanese kanji.
Is it the most efficient way of studying? Would I be better to leave one on the back burner for several years and focus on one? Possibly, but I find this way more fun. The links between them are many and frequently fascinating to me.
Ganbatte! or Ga yau! Add oil as they say in Asia 