Currently I have under cabinet lights that plug-in to the counter-top receptacles.  In the future, I may wish to hardwire these fixtures in a similar configuration.  Normally, you'd install one 24" fixture rather than two 12" fixtures, but I already own these.  Is it code-compliant to hardwire fixtures with factory supplied cords?  The UL stickers on these state that they are listed as "Portable Luminaires", but they're no different than the direct wire models.  Also, I'd prefer to only drill a single hole through the wall.  Can I power the second fixture from the first by making use of the knockouts on the sides, and if so, what are the appropriate connectors to use?  Should the knockouts on the two fixtures be attached with a connector, and if so, what type?  I'm new to permanent wiring.

this what I would do, get a Step drill half Inch so you can install a nipple and two lock nuts to join
both fixture together. from there do the same on the back side on far right the fixture. and drill a hole in the wall and run a romex from the outlet up the wall. and connect up your lights inside the ballast channel.
 it would make it appear neater with out the zip cord hanging all over.

There are already knockouts on the sides of the fixtures.  Can those be used to join the two fixtures?  I don't know if I'll try to install them myself or hire a professional.  I could probably handle the one closest to the power supply, but I don't know about fishing the wire around to the rest of the cabinets.  I ask about code compliance because if I do bring in an electrician, they're not going to violate code.  I read somewhere that fixtures should only be linked together if they're approved for use as a raceway.  I'm not sure if that's true of these.  Also I read that you're not supposed to tap into the small appliance circuit supplying the outlets for lighting.

Fixtures that i seen which are meant to be suspended on chains, have in the ends 2 small holes for the chain and a knockout for the wire. The small holes can be used to screw the fxtures together with bolts/nuts if that helps, to get 1 rigid part and not 2 independant fixtures wired from each other

Tapping from th outlets won['t be a problem as far as physics go (the lamps are very small load, negligible compated to any other load the sockets can be used for), but i dont know what the code says about that

What may be more issue is if it is allowed to do permanently-energized wiring over cabinets. If no, perhaps you can get around the code by adding a receptacle high up in the wall (hidden from plain view by the fixtures), and connect the fixtures to it with a plug

Im not familiar with USA electrical code so im just shooting up what makes sense to me

I have undercupboard lights and they are the kind that are designed to be hard wired. The manufacture (Linogène) sells the same model with a cord and plug, though. The circuit that feeds them is separate from the plugs. I don't believe the code allows a single circuit to feed both lighting and plugs. Mine were installed by a professionnal electrician when the house was built.

As for modifying the existing portable fixtures for hardwiring, my initial guess would be that code does not allow this, but I really don't know.

Hardwiring the plug-in unit might invalidate the UL certification, which in turn might be a code violation, but I'm not sure.  Professional installation can be expensive while fixtures are relatively cheap, so if there is any doubt it may be wise to simply buy new fixtures.  They'd probably fit better since I'd be choosing the lengths that are best suited for the cabinets.  Good preheat fixtures are harder to find these days, but I could probably locate some if I needed to.

Not really sure what the end goal is here, but if you are a novice...keep some things in mind...
-Running wire through knockouts- use the appropriate fitting within the knockout to avoid cutting into insulation.
-use wire rated to withstand ballast temps (do NOT run ROMEX alongside of a ballast)
-Ground everything properly.