I find that once a reader is subscribed on one platform they won't resubscribe on a second... I have pretty different reader bases on Tapas, Webtoon and Smackjeeves.
Readers will generally stick to one site. The readers you get on tapas are gonna be unique to this site 90% of the time.
It makes sense though - if you've already got a subscription on one site why make a whole new account just for that comic? Most people prefer to keep all their subscriptions on one site.
Your tapas mirror will probably only bring in readers who are already on tapas, which means it'll take a while to build up the reader base to the same standard as your existing sites!
There's not really any work around for that (not that I know of anyway). It's just the nature of social media and such.
Hope that's helpful; I'm no expert this is just from my own experience of posting comics and art online for several years.