If it were me I would keep the sans in a smaller pot & bury it into the larger pot, sans roots tend to like close quarters, as to what companion plant I'm sure others will have some great ideas.
There is a sansieveria forum on this board in case you didnt know.
http://forums.gardenweb.com/forums/sansevieria/

In my opinion, one of the great things about Sansevieria is its unique vertical architecture. I think that it makes a striking stand-alone plant.

Another thing to consider with this plant is its pretty aggressive rhizomatous growth. I'd think that they would crowd out any other pot mates.

That being said, will you keep the plant in a sunny location or one that is shadier. Sansevieria does well in either situation but knowing your placement is important information for us to know, if you want suggestions.

Thank you for your replies. I didn't know Sansieveria likes a smaller space -- the idea of a small pot within a large one sounds good! As for light, I'd like it in a location that gets bright indirect light part of the day and then shade, so a shady location overall. I do like the stand-alone look of most Sans I've seen, but I really want to have a diverse look for a large pot I have in mind, so I'd love any suggestions for companions! I will also check out the Sansevieria forum.

There aren't lots of plants w/ similar requirements that would go well together w/ these.

I grow these (& also belong to the Sans Forum), except maybe a ZZ plant which is also tall & mostly vertical, so sounds like other than what you wanted. Sorry.

The problem, as I see it, is that the Sans will require very, very infrequent waterings if grown in lower light conditions. I can't think of a suitable pot mate.

What would you think about looking at other Sansevieria species and cultivars? They come in a wide variety of forms. The Sans Forum will be able to help.

If you have a spot for more light for your Sans, I'd recommend that overall. That said, I've put several different plants with Sans, almost all of my plants are in community pots. Begonias, Tradescantia pallida and zebrina, Callisia fragrans are lovely companions for Sans. Callisia repens is nice but takes a bit more patience propagating tips to get a pot full, with plenty of strands flowing over the edges.

The more plants you have in a pot, the more quickly they use water, so I think it's a good reason to have companion/group pots. If, like me, you repot plants every year or two, you can re-arrange them as needed. The only 'rule' I use is that they must appreciate the same amount of light.

The pot on the right has Sans, a cane Begonia, Tradescantia pallida. Soon this group needs a bigger pot. I like it, I'll be keeping these together during this years' repotting. Try to ignore the sand, when plants are sitting on the ground here under the trees, sand splashes on the pots when it rains. Decided to put this one up on a table where I can see it better from a kitchen window. We just moved here and are working on replacing almost everything outside.

I would put it together with a Pothos.

Some sansevieria are shorter. Inquire on the Sansevieria Forum about putting a Hahnii in the same pot with a taller one.

i've had hen and chicks indoors for many years. they do quite well too. no watering necessary :). multiply like crazy. you can just drop them between sansi stems on gravel and they will root and be ok.
also burro tail (succulents) or smth similar. provided you keep them in a sunny window. or at least east/west part-sun.

Thank you for all the replies and info. That helps a lot! I'll look into the shorter forms of Sans. Will also see if I can find another place for it that gets more light, so I have more options for companions.

Plant mixing in a single pot means survival of the fittest The Sans you want to mix anouther plant with has far better odds of being the more dominating plant.

Tillandsia.

Neoregeliia, Nolina, Crassula will all work well.

Like this trough.