If they are both male you don't realy have much choice but to somehow seperate them. If you don't have another tank to put them in, re-home them or give them to an LFS. If you can already see them sparring and there's obviously some aggression there, they are almost guaranteed to end up killing each other (or more like the big one will kill the smaller one).
However, though you say you can't afford a new tank, paradisefish are suprisingly cheap to keep. They don't grow big (about 3" - not counting fin/extensions), breathe air so don't realy need a heavily oxygenated tank (thus, provided you don't over-feed and are careful to use gravel from a mature tank, you don't need a filter), are remarkably hardy and can actualy do fine in quite low temps (they don't need a heater if your temp. doesn't drop below about 67 deg F - they can actualy manage considerably lower - even up to 59 deg F though this obviously isn't great for their health). Basicaly, all you need to give one of the two paradisefish a new home is a relatively large plastic food storage box (prefferably holding about 10 gallons or more but it'll be ok in a little less if necessary). Make sure it's food-safe so you don't poison the fish, use gravel from a mature tank as a substrate and monitor water params while the fish is in there (it'll cycle like any ordinary tank but the balance will be a little more fragile to keep). Adding lots of fast-growing plants will help keep water quality up but keep in mind that you won't be able to have any lght fixtures so they must be a low-light species. BTW, don't position the make-shift tank in direct sunlight as you'll get major temperature fluctuations. You'll also want some form of a lid (make sure air can get in freely or the fish will suffocate) as gouramies often jump. Including a small ceramic cave or pot will make the fish feel more at home. You may find (if the volume is small) that you need to do more than one weekly water change to keep water quality good but, usualy, provided you don't over-feed, use mature gravel to start with, gravel vac weekly and carefuly acclimatize the fish to the new temp. to begin with, everything should be fine.