Tie lines are the correct choice here.
Since you will have Dante cards installed in both consoles, you can easily use the Dante network to connect the two systems together. You use tielines in and out of each consoles Dante I/O card and then you will also have to patch the audio paths in the Dante Controller software as well.
The only other solution would be to add a second GigaAce card to the Mixrack and connect the SQ console using that connection but since you are already using the built in SLink port for a DX box, it would require an additonal SLink card in the SQ as well so it’s probably not worth the extra expense. Still, compared to using Dante this would be easier to set up, it would be more reliable, and it would have lower latency. You would still use tie lines, but you would patch the tie lines in and out of the Mixrack’s GigaAce I/O card and in and out of the SLink I/O card on the SQ console.
As noted by bolupona, you’ll want to dial in preamp gain settings very quickly and then not touch them again during a show. Each system will have a “trim” adjustment that they should use to make any “gain” changes to their inputs. Otherwise if someone changes the preamp value on the DLive system, it will also change the overall gain level of that input on the SQ system. The only time there is automatic gain compensation available is when you connect two DLive Mixracks together.
.
