So before I could even try this idea, I need to locate how I could go across an international border via sand dune with 2 major requirements:
First, it has to be legal: it's all well and good saying you'll run over the border then come back to your original country but if the authorities knew who you were, and in future you wanted to visit said country, you might end up having a visa that's denied; which a few days ago, former footballer David Icke had his Australian visa revoked (Not crossing a border illegally but getting a visa denied). The Guardian. 2019. Conspiracy theorist David Icke hits back after Australia revokes visa | News | The Guardian. [ONLINE] Available at: https://www.theguardian.com/news/2019/feb/20/conspiracy-theorist-david-icke-hits-back-after-australia-revokes-visa. [Accessed 22 February 2019].
You could cross the border for example from Oman to Saudi Arabia via the Empty Quarter on a sandboard but even there, a huge border fence is installed so crossing through the desert is essentially impossible.
Secondly, it has to be on a sand dune so it can't be in the middle of two countries that have river borders (i.e. A lot of parts between Spain and Portugal). But it has to be an actual sand dune crossing the boundary, for this reason I won't include the Bray dunes on the border between France and Belgium partly because the dunes are way too short and flat to even pick up even 1kph or even do a simple trick, but mostly because the actual border is more than about 20 metres from dunes; but even more so, if you look on Google Maps, you'll find that a footpath on the Belgian side is able to view on Google Street View and if you look hard enough, you'll even see that some bright spark decided to put a fence separating the 2 countries (A pre-EU move, perhaps?)
And finally, in addition to the first point; it makes it easier if the border is open. Last summer, I drove to Germany from the UK and on the way I passed the little towns of Baarle-Hertog in Belgium and Baarle-Nassau in the Netherlands, and thanks to the European Union's Schengen agreement, international borders look like this:
|"House Shared By Two Countries" by Jack Soley is licensed under CC BY-SA 3.0|
It's not an essential requirement for it to work but for some countries, it's a lot better this way than having to apply for 1 or more visas, and potentially getting caught if you cross the border illegally (which I do not condone). If you want to legally cross into another country by sandboard, you're left with any state in the European Union, or in vast dry, arid, uninhabitable desert; but even then border guards regularly patrol the area.
Aside from the small dunes on the Belgium-France border, there are also slightly larger but still quite flat dunes on the border of Lithuania and Latvia, both European Union states and with an open border, and if you want rocky and not very sandy dunes, you could try the France-Spain border. Given the unpredictability of sand dunes and their constant shifting, soon we could even find the dunes could rise and if governments decide not to update their borders, we could see a sandboarding session go international very soon!
If you want my opinion about this, you're better off crossing states and provinces rather than an international border mainly because of all the trouble you'll avoid and big countries such as the United States that not only have many states, but also many dunes, will make this venture a lot easier; even if you cross a border on Google Maps, you might even still be in the same country, state, province, etc... that you started in.
Also, a side note on an old post; the 4 country border of Namibia, Zimbabwe, Botswana, and Zambia no longer exists; it is not a quadripoint, it is 3 tri-points separates by a clean border between Zambia and Botswana. "Perhaps the world’s most notable tripoints fall in the middle of the Zambezi River, near the borders of Botswana, Namibia, Zambia, and Zimbabwe in southern Africa. The area features a unique “double tripoint,” with the two tripoints separated by about 150 meters (490 feet)." - Double Tripoint in Southern Africa. 2019. Double Tripoint in Southern Africa. [ONLINE] Available at: https://earthobservatory.nasa.gov/images/85867/double-tripoint-in-southern-africa. [Accessed 23 December 2019].