Hi Geraldo, I've to listen again to the guitar parts to help you but anyway here are some more hints :
You should give up with the midi bass and grab a single coil guitar ,as a guitarist you can easily play this instrument that has 2 strings less , the opposite is not true . I'm not a bassist but I often add very simple bassline played on my strat or tele with a fender profile. I can send you a bass rig if needed. Here is my method : dial up a clean profile set the harmonizer to -12 or -1 octave ( harmonizers tracks better than the transpose for me ) EQ the highs down and actually play those bass parts and feel like Jaco Pastorius ( I play those parts without pick, try to play fewer notes than on guitar and give some groove ).
Lsiten to these bassline examples , much more musical & fun to play to me than a midi bass and took like 5 minutes to record ( https://soundcloud.com/waraba/jazzbox-vs-drz & https://soundcloud.com/waraba/sitar-hero-v2 )
Regarding the double tracking I prefer playing another track instead of simply pasting & reamping a single track , it sounds more musical to me. About the sound it's better if the 2 tracks have really different tones ( Les Paul + strat , Jazzbox + tele ) It's mainly a matter of 'contrast' , pan & EQ , the objective is to hear like 2 different guitarists like you can hear them on a stage. Rigs can be the same or not, the guitar will make the differences.
You should wait for your keyboardist , he will arrange the whole song and new ideas will show up as the track gets more elaborate since you'll have more harmonies to stand on.
Remember to leave some room for the singer and you solos.
I agree with Jamstix , it will feel and sound more realistic if you use it on your track.