How can I split bus in Quartus?

The problem is that the 2D bus coming out int8 is getting separated to single wires when they connect to nios_system. When you connect those buses to nios_system use the bus drawing option (thick line) because right now you are trying to funnel 12 wires into 1 which is why Quartus is complaining.

I have never tried round 2D bus lines in a schematic before so I'm not certain what indexing you need to use so you might need to use adc[0][11..0], adc[1][11..0], etc... when connecting to nios_system.