When I first seam sealed my TT Stratospire 2, I was a bit concerned I over diluted the silicone/spirits mix, as the tent fly looked the same as before I began. Also the stripes I painted on the floor were so thin that, while visible, were so thin they provided little to no grip.
However, the tent remained perfectly waterproof through an intense, night long thunderstorm.
So, I doubt a 2nd coating is necessary. If you're really worried about it, just bust out the garden hose and see how waterproof she really is :)