This is actually a question I would like to know the answer to, myself. That said, it's fairly strangely worded.

I'm gonna go with the Bassman again. Nice bass tones and maybe you want to find a Square Wave Distortion pedal to tweak those highs. Either that or a Trace Elliott, but those sound pretty artificial to me.

The only amp that I've ever had success running both guitar and bass through would be this ancient Kustom combo my friend has. Neither of which sounded anything special. But there was alot of unavoidable high-end and some raunch to boot. You might want to try just finding a head that fits the bill for "your sound" and just experiment w/ cabinets. I own a weird Roland Jazz Chorus head of some kind that seems to take to bass/guitar/vocals in a pretty original way. Just run through a generic cabinet. The bassman (previously mentioned) is notorious for getting a decent guitar sound for being a bass amp... so go figure. That is, if we are talking about the same bassman.

Ampeg Portaflex sb-12 is a nice bass amp that sounds good for guitar as well. Not too costly either.

-Amos

ampeg v4 or v4b

Traynor YBA-3.

I know I'm reitterating here, but:

Ampeg V4 with reverb (you can turn off the reverb for bass, comes in handy for guitar) both versions were the same, they added reverb for guitar -- Super Loud
Kustom 200B -- sounds better and louder with a 4 ohm load (also of note, with guitar it actually sounds better using bass speakers as opposed to a guitar cab).
Fender Bassman silverface or older

I would avoid any cabinets with 15s if you plan on getting usable guitar sounds. Opt instead for 10s or 12s.

I play in a band where I try to fill up the bass frequencies with my guitar sound and I use a Marshall 50 watt JCM800 bass head and a 4 x 12 Sound City cabinet with Fayne (sp?) speakers. It works pretty well. We're two guitars and drums.

Sounds like this: http://www.sicbay.com/mp3/spazz2.mp3

Sorry if this seems lame to post my band's song. Oh well.

Hiwatt 200!!!