yes you can do it. There are various ways. The best way is to get a White Industries Excentric Hub and probably one of their sweet freewheels too. The more normal way is to buy a cog and spacers, or tear apart your cassette and make spacers from PVC pipe or any combination of that, use the spacers to adjust chainline, shorten your chain as much as possible, get one of the many tensioners, paul, surly, dmr, etc. and have at it. If you have a freewheel hub, you may have to do some math figure out your chainline for the front and respace and redish your rear wheel. Not a big deal if you know how. Either way. It can cost as little as the pipe for spacers if you're lucky.
You can use your existing rear derailleur as a tensioner, and you can just get rid of the front derailleur, it's useless.
The advantage is simplicity. That's it. No chain slap, no gear shifting, no chain suck, no ghost shifts, no bent derailleurs, no broken shift cables, no questions. If set up properly they are very reliable, no nonsense. And when I say set up properly, on a bike with vertical dropouts it's all in chainline. The tensioner makes up the rest.
A normal XC gearing for a 26" wheel is 2:1, i.e. 32,16