Not really
No it's not since the 48V charger won't recognise the 24V battery as the battery. The 48V charger has a levels of voltage set up to recognise the battery. The lower one might be set up somewhere around 44V and the max one around 53V.
You would need another 24V battery and connect them together in the way that both give you 48V and then try to charge, Please be however aware that it's best to use the original 24V charger. We at Oxygen do not allow to use any other chargers that are not approved by Safty Energy (our battery manufacturer) since the selection of wrong charger might cause explosion or at least damage the battery
Hope that helps
Andrew
Oxygen Bicycles