LTSpice predefined components for ideal Voltage Gain and Sum
An ideal voltage gain is implemented with a VCVS (voltage-controlled voltage source). You specify the two terminals of the controlling (input) voltage and a single gain parameter.

You can combine two voltage sources just by putting them in series. If that's not the configuration you need, use two VCVS that are controlled by the two input voltages, each with a gain of 1.0. Then place the two VCVS in series.

This is a pretty standard technique for any SPICE simulation.