2 x 8-channel multiplexers with with linked outputs
Yes, as you suspect.
The voltages from S0a and S0b would be connected together by the low impedance paths through each enabled mux. Unless these connected voltages are equal, they will cause a current to flow between them. The current value depends on each voltage source's internal resistance and the sum resistance of the two enabled mux channels, which you can get from the mux datasheet. If this current flow is high enough, it may cause destructive power dissipation in your mux or voltage source components. It will also cause the voltage seen by the op-amp to be somewhere between the two voltage source levels.