I would never have two bass lines playing at the same time. They are no different from other melodies in a track. If you have two of them playing at the same time you can argue that you don’t have your phrasing right. You wouldn’t have your two main melodies playing at the same time would you? You might have second / sporting melodies or synth lines interacting where they work together, but not your bass lines, they are too distinctive to the track, and rarely work together. Just think of them as elements that shouldn’t me mixed. This is all personal opinion.
So er, frequencies, yeah… I’m not a producer and my knowledge of some of this is sketchy at best, so I might be talking out of my hoop here… Bass lines in prog and other electronic music all occupy a similar place on the frequency spectrum, and more importantly all sound fairly similar, have similar ‘voices’ and qualities (in my opinion obvs), so if you play them at the same time, they just sound very muddy and a bit of a mess, you can’t differentiate one from the other very well, so, erm, why bother? If you play two main melodies at the same time, one a classic piano sound / voice and the other a very electronic synth wave sound / voice, they might also occupy a similar place on the frequency spectrum, but you could differentiate one from the other due to the very different qualities of the sound, and they probably wouldn’t sound too bad.
Next, volume, and again, I might be chatting baubles here… This bit is about kick drums and bass lines, we shall just refer to both of them as the collective ‘bass’… Because the bass on track A and track B occupy similar frequencies, if you play them at the same time, your doubling the amount off bass output. You’ve just listened to nine and a half minutes of beautifully balanced bass, mid and treble on some obscure epic house track, and then, whamo! Double the bass! In a club you would notice it as being fucking loud and or a clunky as fuck mix, and on a recording it would just fuck with your overall levels. So… why doesn’t the same happen with the mids and the highs? Well, it can, but in my opinion it’s less noticeable. Probably something to do with the human ear and ‘perceivable sound’ vs ‘actual volume’. Also, if you play two similar kick drums at the same time, you will encounter all manner of volume issues and you may get the dreaded ‘phasing’ where the bass lines cancel each other out, and disappear into a flat nothing sound with a shitty clipping sound to boot!
*I don’t produce, I haven’t played out in a long time, still do mixes online though, could be talking broken biscuits, just starting my 4th beer.