Push-pull 21s - how do I check both cones are moving together before I close the box up?

The battery test works fine, that's exactly how I sorted mine. Touch the positive battery lead to the positive terminal and the cone jumps outward. Do that with each driver and mark the frame with a paint pen so you're not guessing later. One catch with a manifold build: 'outward' for the rear driver means toward the back of the cabinet, not toward the manifold. Both cones need to move in the same physical direction at the same moment, so one of them ends up wired opposite at the amp terminals. Get your marks right and the rest is just following through.

If you can, do the battery test before the drivers go in, or at least reach in and mark both frames while the baffle is still off. Fishing around behind the internal braces later with a battery in one hand is misery. A multimeter works too if the drivers have a datasheet that says which lug is which, but the battery jump is more trustworthy since you're seeing the actual cone, not trusting a label.

As for getting it wrong: you won't hear it as a obvious buzz or rattle, you'll just get this weak, hollow low end that never fills the room no matter how much power you feed it. The two drivers fight each other and the output cancels right where the horn is supposed to do its work. I've seen people chase port noise, leaks, amp gain, all of it, when the actual problem was one driver backwards. Some guys catch it by playing a low sine sweep and hovering a hand over each cone, they'll be moving opposite each other, but by then the baffle's ussually on. So do it now while everything's open. Wire one driver flipped relative to the other per your marks, then after it's sealed up, run a sine tone near the low end and confirm both cones are moving togather. Five minutes of checking beats pulling a baffle screw fourteen of them.

One thing I'd add from my own box, which is that factory terminal markings are not always trustworthy. I built something similar a couple years back and one of my drivers had the lugs labeled backwards compared to its twin, same model, same batch even. The datasheet said the big lug was plus on both of them, but the cones jumped opposite directions when I fed them each the same battery, and that was the whole story right there. So whatever the labels or the paperwork claim, trust the cone and nothing else. I also write which wire goes where on the wood with a pencil next to the terminal hole, because my memory has never once survived the time it takes to cut a baffle.

Been doing the battery trick since back when we were soldering leads onto drivers for PA bins in the 80s. One tip from those days: use a plain 1.5V cell, not a 9V block. The cone jump is smaller but gentler, and you can hold it there a second to be sure wich way it went. Old trick if the jump is hard to see, lay a strip of tissue paper across the cone and watch which way it kicks. Cheap and it works even in bad light with a rear driver buried behind braces.

Long alligator clip leads saved me on my manifold build. I could clip the cell to the lugs, let go of everything, and step back and watch the cone at eye level instead of crouching with my nose against the brace wondering if I imagined the jump. Cheap little things, like the kind sold for hooking up test gear, and I still use them for every driver I touch. Also worth doing: once the wiring is in but before the baffle goes on, clip the battery to the terminals where the amp wires land. That checks the whole path including your solder joints, not just the drivers by themselves. Caught a cold joint that way once.

This thread is clearing up a lot for me becasue I did a pair of speakers last year and the bass always felt thin, now I'm wondering if I had this exact problem and just blamed the room. One thing I'm still fuzzy on though: when you flip one driver's wiring, does that flipping happen at the speaker's own terminals, or can you just swap the two wires where they connect at the box plug instead? Like, is it the same result no matter which end of the wire you cross them at? Asking because reaching into my cabinet was the part I hated most and I'd rather do the swap somewhere easy to get at.