They are fixed fuel tank vents so why would you need two (unless you've got two fixed tanks?).   If you are using them to ventilate the hull then they won't work unless on the floor (see below re: vapour weight) - leaving hull drain open will do a better job. 
If used as a tank vent they will let rainwater in (normally you need a loop in the vent pipe to catch water which hopefully evaporates again before it gets to the fuel).  If you are fitting a fixed tank then its better to use a combined filler/vent fitting. 

Yes one for each side 

A pair?

I've ordered a pair of those vents from Aliexpress,  Less than a tenner for the pair including delivery.

I use that method. Be aware thermal expansion can cause neat petrol to be forced out or sloshed!

You could install a vent easily

'Strong smell' means you've got an open tank or fuel line (if the tanks vents are closes there should be no smell at all).  The diesel heater poses no risk at all as the combustion system is only exposed to the outside of the cabin  (provided you install it through panel as it's  designed).
You could install fixed fuel tank (30l would be plenty) with external fill and vent pipes - downside is you have to carry and empty fuel can to be able to refuel on the go.  Or fit an air vent above the waterline someplace as low as possible (fuel vapour is heavier than air) and use a camp bed so your head is level/above the vent if camping (which I assume is the issue otherwise there is plenty ventilation in a hovercraft when moving).

Looking for ideas on how to vent to the outside of my hover cabin, as when I open the soft top side screen, I am always met with a strong smell of petrol fumes.  It's always at the back of my mind that a spark from the isolator switch or one of the other switches could cause an explosion.  Now that I'm about to fit a diesel heater, is another risk factor.  I know the risk is minimal, but it would be good to minimise that risk further


I know one option would be to move the fuel tanks to the outside of the craft, but I would rather avoid that if I can, as that would probably upset the balance side to side as one tank became empty and the other tank full.  Plus it would make it awkward connecting the other tank if it ran out of fuel while on the water.