I'll answer Jeff's question first,
Wouldn't using one bulb for longer be better? Obviously I'm no pro at this but I did go looking up light exposure for tropical areas and the average is around 12 hours all year round. I would imagine that running your single out longer, even 10 hours, might be a better option if two for 7 hours is too much.
No. The issue is two-fold, intensity and duration. And a weakness in the intensity cannot be "made up" by lengthening the duration--this will only increase algae. Plants need a specific level of light intensity (brightness) in order to be able to photosynthesize; this varies with species, as low-light plants like Anubias require much less light intensity that would Cabomba. This is why I always recommend knowing the plant species intended, and then deciding on light [if starting from scratch], or if one has a set fixture, then trying plants most likely to manage with that intensity. As a gardener, I can attest that the very same principle operates with respect to sunlight/shade for terrestrial plants. I'll come back to duration below.
Now to smit's question. I don't remember the specifics, and to be honest my thinking has changed a bit over the past couple of years; taking your first example, with one T8 48-inch tube over a 4-foot 55g tank, I would expect the swords (green-leaf species, not the red-leaf) to be OK [mine were way back], and floating Water Sprite would be fine. But substrate WS would not manage, nor would Vallisneria. Brazilian Pennywort might, certainly floating it would, but I rather think planted in the substrate it would fail.
Having had a 55g years ago with one T12 tube, and now having a 70g (same length but wider) with 2 T8 tubes, I would go with two T8 tubes over any 4-foot tank. It just opens the options for plants. I have to be careful with algae in my 70g, but there are a couple of ways to deal with this. One is with lots of floating plants. Forest fish without any question are "happier" with a roof above them. I've done a fair bit of experimenting with this, and the results over several years are obvious. So this solves two issues.
But returning to duration, that is the other control we have, although it works within a somewhat narrow framework. If you have too much light over the tank, no lessening of the duration will compensate. The plants are going to photosynthesize full-out (provided all nutrients are available to them) but with too much intensity you are almost bound to have a shortage among the nutrients, and usually it is carbon first and then probably nitrogen (ammonia/ammonium) since adding the others is relatively easy with fertilizers. George Farmer, who writes on planted tanks extensively in the UK for periodicals like PFK, says that six hours is about the minimum photo period for aquarium plants. I run my tanks at 8 hours now, and have for a couple years, having reduced back to this from higher periods due to increasing brush algae. It is ironic that back in the 1990's, with my current tanks and lighting, I ran them 15 hours and I saw no algae issue except for a couple of times. I was fertilizing less then too. Obviously the balance I had then is not with me now. And I have cut back to once weekly for several months but increased to twice weekly (liquid fert) because the plants clearly began to fail. Sometimes finding the right balance at which plants will thrive but algae will not can take some experimenting.
The issue of too much intensity is the main problem with T5 HO tubes. If one is not adding diffused CO2, having two T5 HO tubes over a 4-foot 55g, 70g, or even 90g tank is almost guaranteed to be too much. I have tried this, for one full week; the two 4-foot T5 HO tubes were even too much over my 5-foot 115g tank, and I returned them for two 4-foot T8 which works fine. But I keep to specific plant species; I know some would never make it, but my concern first is my fish, not having a water garden.