The description for a pair of enviando pedras diz:
Once sending is cast through the stones, they can't be used again until the next dawn.
However, if the sending stones are far apart, dawn may occur at a different time for each stone. In this case, when do the stones recharge, and how often? What if one of the stones is on a different plane, on which time may not pass in the same way as the material plane? (Assume this other plane does have a dawn, or a reasonable equivalent.)
From the sidebar on page 144 of Guia de Xanathar para tudo:
RECHARGING WITHOUT A DAWN
Some magic items can be used a limited number of times but are recharged by the arrival of dawn. What if you're on a plane of existence that lacks anything resembling dawn? The DM should choose a time every 24 hours when such magic items recharge on that plane of existence. Even on a world that experiences dawn each day, the DM is free to choose a different time-perhaps noon, sunset, or midnight- when certain magic items recharge.
So the point in the day-cycle called alvorecer by magic item descriptions is (or can be) any point chosen by the DM. For multiple timezones (if their world has such) they may choose some equivalent to Greenwich dawn, (maybe an observatory outside Waterdeep?), and in the absence of time(zones); my preferred option is generally that it's some point during that long rest, just to make all the accounting easier. For different time rates the same goes; the DM chooses a point/"dawn". Saying that each stone needs to experience their local dawn or that the one the PCs are currently experiences seem like two reasonable options, but salt and pepper to preference.
They recharge each dawn, which is the mechanical condition that resets the stone. A dawn always creates one recharge, and there can't be stored more than one charge.
So it really depends on how many dawns happen. If one Sending Stone is in a plane that experience dawn every 5 minutes, then that satisfies the mechanical condition that would enable the restoration of the charge if it was used.
Once sending is cast through the stones, they can't be used again until the next dawn. (DMG 199)
XGtE offers the optional rule that a DM may use to change this mechanic on when the Sending Stones recharge. If you use this optional rule, a Sending Stone will recharge once every 24-hour cycle on that plane. Different planes can have differing progressions of time in the multiverse, see for example the Feywild (DMG 50).
The DM should choose a time every 24 hours when such magic items recharge on that plane of existence. (XGtE 144)
Thusly whether or not you use this optional rule, it is possible that the Sending Stone on one plane will recharge the pair so often that on the other plane you will be able to use it many times during one 24-hour cycle.
To every other aspect of the question: ask your DM who creates the world (DMG 4).
There are actually two answers to this, though obviously I prefer the above. One answer is by rules as written (RAW), and the other is by rules as intended (RAI):
RAW: They will recharge when either stone experiences dawn.
The exact time that dawn occurs depends on your location. It can be assumed that the location of the item determines when dawn is for that item. For most magic items, this is fairly easy to adjudicate, since there is only one dawn per day for any given location. Sending stones, however can be in multiple locations while still being one magic item.
The simplest interpretation is that the stones recharge twice, once when the first stone experiences dawn, and then again when the second stone experiences dawn. This could be seconds apart, or minutes, or hours.
RAI: They recharge every ~24 hours.
Looking at designer intent, nearly every single magic item recharges at an interval of 24 hours, whether it's dawn, dusk, midnight, etc. Most likely, the designers didn't anticipate any of the complexities that come from a strict interpretation of "dawn" and such, but rather meant it as just a bit of flavor to make magic items seem cooler. They could just as easily have said that all of them recharge when you finish a long rest, but that breaks immersion and is less mystical.
To support this, imagine what would happen if a strict definition of "recharge at dawn" was taken. Any magic items that recharge at dawn could, in theory, be recharged multiple times per day, if the user is atop a hill or mountain. Simply let the item recharge when the sun hits the top of the mountain, use it, and then race down the mountain to a point that has yet to be reached by the sun. When the sun dawns upon that location, the item recharges, and can be used again. Race further down the mountain, and repeat until you run out of dawns to recharge your item with.
Even without a mountain, if you can move faster than the earth revolves. At or near the equator, this would require absurd speeds, however near the poles dawn "travels" slowly, approaching zero speed, and anyone could recharge their items over and over again by simply walking ahead of "dawn".
Since such a situation would be absurd, a restriction needs to be put in place to prevent such exploits. Given that the developer's assumption seems to be that dawn will only happen every 24 hours, then the logical way to limit this is to have magic items recharge only if the condition is met, e ~24 hours have passed.
Tags dnd-5e itens mágicos tempo