Hello Petro, welcome to the Forum.
In terms of strength, the magnets inside the rod ends are the same, with the same magnetic strength.
Regarding the length, two short rods don't equal a long rod. This is not due to some magic mathematical ratio but simply due to the requirements of the EN-71 and ASTM toy safety regulations of the EU and US respectively.
Forum member Karl Horton has posted the lengths and some thoughts here :
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http://forum.geomagworld.com/viewtopic.php?f=6&t=10 ]
PolyClare has done some experimenting with a combination of KIDS Panels and Pro, but not directly combining the two lengths of rods.
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http://forum.geomagworld.com/viewtopic.php?f=4&t=55 ]