Currently, it does support blocking calls on all SIM cards, it's just unable to differentiate between them. If you want to treat them differently, it's not possible yet.

Some other apps support this because they are either:

• The default phone app
  This app isn't a phone app.
• They use the CallStateReceiver
  That would break some features, the lockscreen will be turned on, preventing silent call rejection or silencing(ringing at background). And this approach would significantly increase complexity (code, UI, user experience)

And it doesn't seem to be that useful, if your rules work well for SIM 1, it should work well for SIM 2 as well, unless you want to hide all notifications for SIM 2.

Therefore, I prefer not support it for now.

I've submited a feature request to google (link), maybe give a "+1" to it. Once supported, there will be a SIM option in the regex rule.