In my experience you'll be in trouble with any shelter if water comes actively flowing in, at all. A bathtub floor will wet through with standing water underneath. Avoiding this is part of good site selection.
What you might see with a floorless shelter is hard rain, or more likely hail, boucing in under the sides if the precip is especially vociferous. As Travis noted, the TS has enough floor space that some might get under, but you'll still have plenty of dry space for two.