You can't use the stacking ports no - you'll need to use a standard link (or two, as a LAG) between the switches.
The SFP+ slots obviously provide the best options for high-capacity between the two - maybe using DAC: MA-CBL-TA-1M (3M also available). Full list here: https://documentation.meraki.com/General_Administration/Cross-Platform_Content/Small-Form_Factor_Plu...
you could also just use some copper ports and some patch leads, if high capacity (10 Gbps+) between the switches isn't necessary.
Make sure your STP setup, in terms of root bridge, matches your VRRP config.
I take it that, by two port mirrors, you mean two different destination ports for mirrors?