If you're talking about a mod (20"), are you sure you don't have a 24/18 setup?
Either way, that's pretty low geared - even for a trials bike.
Having two freewheels isn't ever recommended, however you can get a perfectly functioning drive by using two freewheels providing one is glued so it can no longer spin. If you don't do that, you may find that when you goto do something like a pedal kick, one freewheel has to engage, and then the other making it feel like your engagement points are virtually non existent.
But the main problem to this is that they don't make freewheels small enough. If you were to put a freewheel on the rear (and have your 18t freewheel on the cranks) you'd have to find a 12t-14t freehweel which i don't think exist.
However, if you put your 18 tooth freewheel on the cranks, you can simply buy a 12t fixed cog and screw that onto the rear hub. This is the system the majority of mod trials riders use.
Or, you could buy a 16t freewheel for the rear and this would make your gear ratio a little bit more towards the norm.