Does anyone have any hints with this? Doing it with a band currently and just wondering whether anyone's got any tips (especially in terms of the bass amp). Any effects or something that bassists might recommend?

Here's what I'm doing:

The Hunted Crows debut EP (released last Sunday 21st September)
Sniff You Out music video:

I am not sure what you mean by "Bass/Guitar Split System?" Are you playing both bass and guitar, and you want some recommendations for your bass rig? Or are you running a split signal from your bass: one goes into a bass rig and the other goes into a guitar rig?

As far as bass effects, most bassists go very light on effects, unless you are playing 1970s-era funk (in which case, you definitely need a good Envelope Filter pedal). You should have a compressor, and depending on how limited your EQ section on your amplifier is, a 10-band Graphic EQ can be a big help.
"Maybe this world is another planet's hell?" - Aldous Huxley
I think you may need to define what you want to accomplish.

If you want to extend the system's range, there are better, lighter ways to do this and have everything in a single 900W cabinet that your girlfriend can single-hand into a venue.

If you want to split the guitar into a couple of bass strings and four standard strings, the clever way to do this is with a Variax (you can tune two strings an octave down without ever changing from E standard in terms of actual tension on the strings). You can actually do this on the fly with a single stomp if you have the Variax connected to a Pod floor model. At that point, you want that lightweight wide range cabinet I mentioned, above.

If you want your rig to look traditional but not carry quite as much crap around, buy the ISP Technologies Vector SL. It's a 600W powered subwoofer designed to strip the lows from your signal and reproduce them through the sub, leaving the mids and highs to use your tube head's power to feed the speakers in your existing 4x12.

If you want to lighten up even further, ISP makes a Vector cabinet that looks like a 4x12, except that it's two separate cabinets hidden in the same form factor. There's a 15" sub with a 400W built-in amp that works like the Vector SL, above, except that it feeds the mids/highs to a pair of ordinary Celestions in the other side of the cabinet. Both methods clean up the bottom end tremendously and give it great definition.

If you want to use the existing system but clean up the bottom, get an external crossover or a splitter. You want to run an HPF (High Pass Filter) before the guitar amp so that it handles only the mids and highs. You want to EQ the bass amp so that it's not trying to recreate the mids/highs in the same amp. Note that you're NOT going to be boosting the bass using shelving controls. You'll probably want an HPF on the bass amp that will cut off anything below about 40-50Hz because your guitar isn't going to send that signal anyway and you don't want to waste power on inaudibles in that area. Boost the 120Hz - 250Hz band a bit and call it a day.

So what is it that you think you want to do?