For some reason, to be "proper"(and I think there might be a law too) your low beams are supposed to switch off when you flip on your highs. (We do own a Honda Accord that has seperate low/high beams and the low beams remain on with the highbeams. However every other make and model of car I've driven maintains the low beams be off even if they're not in the same bulb as the high beams.) I'm not sure what the actual reasoning behind this is, since whenever I have my brights on there are no other cars in sight... so what does it matter how much glare I make?
But looking at the technical aspect of this there are several problems.
1) On bulbs with low and high filaments in the same bulb(9004 for example, very common on many cars) you can risk a possible bulb explosion by either bypassing your wiring or manually holding the high/low selector between settings to keep the low and high beams on at the same time. Now I have to admit that I've done this myself a few times(no more than 15-30 seconds max, my hand gets tired and I worry about things I'll discuss later) and haven't ever experienced a bulb explosion, but I'm sure the life expectancy on those bulbs was affected. If you're buying the premium bulbs discussed in this thread, you want them to last as long as possible, right?
2) Wiring. Wiring is a big issue here. Stock factory automotive wiring is cheesy. Anytime you're pulling more current over these wires than what the OEM bulbs that came with the car pull, you're bound to run into problems. At the very least your problem will be less than optimal output because your wiring just can't carry that much current to the bulb(s). Your wires will heat up and in the worst case you'll have melted wires or even a fire. Don't count on the fuses to save you from melted wires, they're there to protect against short circuits. In most cases the extra draw from your lighting modifications will not be enough to break the fuses, but can be enough to melt the crappy wire. Also, remember that while it may be OK to pull extra current over wire for a few seconds, the longer you do this the more heat will build up and the more trouble you're asking for. If you're gonna make a permenant modification(IE. not just holding the high/low selection between modes) to your lighting, I really recommend you look at your lighting first. I've heard of many people buying bulbs off of ebay, or even "off-road only" bulbs from auto shops and sticking them into the headlamps without taking the proper precautions. Sure, it's fine in the garage after you get them in there, but then you drive around for an hour or two and end up with melted wires and no more light. Imagine this happening at night... Good thing you're flashlight addict and have 5 spare flashlights with you, right?
So, BuddTX, that kit looks pretty sweet and I've heard of some people using it. You should be OK as long as you stick close to OEM wattage bulbs( probably 65W/55W or whatever they may be for your truck). Any bulbs you buy at a reputable auto shop ot website manufactured by a reputable company(Sylvania, etc) are going to meet this criteria(be careful with "off-road only" bulbs though, they're higher wattage).
However although your wiring may be able to handle this without any troubles(no melting, fires, all that nice stuff), be aware that the factory wiring still isn't very good. If you measure the voltage at your battery and then at the wiring harness for the bulb, you'll see what I'm talking about. Even a slight difference in voltage can result in significantly decreased output and whiteness. Also, another thing to keep in mind is that if there's not too much voltage drop now, as that (crappy)wire ages and is exposed to cold and other automotive extremes, it's going to conduct worse and worse and your lighting will be affected. What I'm saying is that another thing you can do if you're really into your lighting is rewire your headlamps with better wire(12ga or lower). This is a whole different topic, but I thought I'd touch upon it briefly.
Anyway, for $19 I'd say go for it if you're interested. It will be nice to have both the lows and brights on at the same time. However I bet you can look up instructions for how to do this for your vehicle and save the $19 for the "kit". It's not that complex and should take you but a few minutes.
Anyway, as always, keep us updated. Sorry it took me so long to reply, I didn't notice this thread was active until I saw it tonight in the "last 50".
-tibim