Nice. So you’ll be going 0 to 100, very exciting.

Depends on the languages you’re learning, I’d guess. Learning two languages that are similar to each other has drawbacks and advantages, and the opposite would be true for very dissimilar ones.
Of course, then you have to factor in possible similarity to already known languages too!
Are we talking about Japanese +1, or 2 entirely new ones, both starting from scratch?
Also, which language(s)?
(Arabic?
)
I get the feeling I should also address the OP. Sorry bout that. 
Personally I tried Kitsun, Torii, and floflo for additional learning, and none of those stuck. I like Kitsun for the layout and the user base, I guess?, Torii for the convenience (vocab is pre-sorted by WK level of the Kanji when applicable, otherwise I think it’s sorted by frequency, and it’s also only vocab you don’t learn on WK <-this actually seems to fit what you’re looking for pretty well? You can also suspend words you already know), and floflo for the possibility to view vocab that appears in a specific text and sort by frequency.
They all have a different focus and will fit different study types and needs. If you try them out, maybe one of them will be it for you. Or not, maybe you already have your ideal study tool. 
For me it’s just that I’m pretty saturated in the SRS field already with WK and the Anki I’m doing. Keeping track of another SRS system became too much of a hassle. Maybe when I’m done with slow down on WK I’ll go back to one of them.