I think this is possible, but not as easy as just connecting them.  with any 2 speakers or speaker cabs, all you have to do is have the impedance (measured in ohms) the same on both cabs, but the bit that's often not obvious is that when you connect 2 cabs together in parallel, the actual impedance that the amp sees is half that of one cab.  so connecting a pair of 16ohm cabs, you would connect them both to the 8ohm outputs on the amp.  but if connecting one 16ohm cab, it goes to the 16ohm output on the amp.

so your 4x12VS-B is capable of being 4 or 16ohm mono (it should say this on the connetion plate on the cab).

Your Spider 4x12 cab I think is capable of 4 ohms mono.

So looking at that, you'd say "well they're both capable of 4 ohms mono, so I'll just connect them to the 2 ohm output"... and there's the issue!  there isn't one.

BUT all I've been talking about is parallel connections of speakers, because that's how just about all amplifier manufacturers internally wire the speaker outputs.  e.g. the two 8 ohm outputs on the Spider Valve head you have are internally wired in parallel.

If you connect a pair of 4 ohm cabs in series then that's a much simpler calculation, and you would add the two together to give you 8 ohms.

Mesa Boogie detail a lot of this in all their amplifier user manuals, although I think they go into too much detail sometimes.  They also talk about being able to mismatch speaker loads, and these mis-match loads are only acceptable on Mesa Boogie amps, not Line 6 (just in case you think of trying it).  But in those guides, Mesa talk about serial connection boxes, which you can actually buy direct form Mesa's factory, and it seems hard to find these elsewhere.  But effectively it's a small box, with 3 jack sockets on it, and a couple of wires inside.  there's nothing very complicated about it, so if you've got a soldering iron and a drill, £10 spare, and less than an hour of free time you could make one.  after you've made it you could also test the impedance with a multimeter and you should see roughly 8 ohms (7 to 9 would be fine, as restance and impedance aren't quite the same thing, so an 8 ohm speaker impedance wont show quite 8 ohms of resistance in most cases and situations).

hope that helps

Rowbi