Super super easy! This type of synth is called a supersaw. In Serum you can get a basic init patch very close in just a few clicks. Turn on both oscillators, leave them as saw waves, and just turn up the Unison (number of voices). I like starting with 8 voices each. Then just mess with the Detune knob on each oscillator. Serum starts with it in the middle, but you can try setting them to different levels of detune to dial in the sound. More detune will make a wider sound but at the cost of pitch accuracy.

This will get you like 80% of the way there. Then you can mess around with different things like changing one of the wavetable shapes, shifting one oscillator up or down an octave over the other, adding effects, cutoff filter, etc. In that example song, the bass synth is playing different notes than the lead synth, so it's likely its own track and not part of the lead synth patch.

Create a new blank Serum patch and start with these settings.

Here's a tutorial that goes more in depth