Regarding getting a fuller sound in a duo bass / drums band.

I think the sound you get in the recordings is great for the compositions, so I don't really know why you feel the need for a fuller sound.

I would call what you are describing multi-amping. You can use multi-amping a lot of different ways, so there is no one answer to your question of does it thicken up the sound...it depends.

Some people who multi-amp simply plug into two amplifiers. The results can be a bit unpredictable any time you start pairing dissimilar speaker systems. IMHO the best reason to use this approach with a bass and guitar amp is if you want to run distortion or some other effect through one amp and clean through the other. Then you can create some interesting sounds and also maintain a nice fat and relatively tight low end.

Guitar amps are generally fine with bass, but most guitar speakers can be easily damaged by bass if you play with very much volume. The amount of caution depends on the speakers you will be using and how loud you plan to play. Running a high pass filter on the guitar amp is a good way to protect the speakers.

Some players like to use an octaver effect to double the frequency of the bass and then apply either distortion or some sort of synthesis. The resultant signal is routed to the guitar amp. The idea here is to create a thicker orchestration by creating tones to fill in the frequency band between the low notes you are playing on the bass and the high notes the guitar is playing. Of course the success and desirability of this would be dependent on the composition and how you choose to score the music.

With this approach, you could definitely thicken up the sound of the band, but IMHO the bass takes on a new role; in essence it evolves into a new instrument. It's very likely your note choices and technique would need to change to support this approach. But I think it would easily fit into the style of your group.

Maybe @Jazz Ad will comment...I think he knows a bit about this subject and may suggest some relevant recordings...here's one of his relevant posts Bi-amping, guitar head, p.a. speakers...

 

If the signal to the guitar amp is 1 octave up that will help protect the speakers, but an HPF may still a good idea if the speakers don't handle bass very well.

I would encourage you to try whatever effects you are interested in on the octave. You can definitely create some cool sounds.