Fireborn: Dark Phoenix
The twinned continents of North and South Encyron are linked at their midpoints by three land bridges, which are 16, 7 and 0.6 miles across, each. South Encyron was initially colonised by trade ships following coastal routes to the far east towards plantations and slave trade posts in Pelargossia; many of these colonies were beyond the ability of the Empire to enforce military rule on, and a wave of secessions (and arbitrary taxation on trade) helped these would-be Kings to grow their nascent Kingdoms for centuries before the Imperial navy was powerful enough that an attempt to re-unify was possible.
North Encyron was colonised far more recently by a further wave of secessionists, this one largely comprising colonists fleeing Imperial rule of South Encyron, during the subjugation of the nation of Encyronia. The narrow land bridges helped ensure that even relatively few defenders could easily hold them, and sea attacks were the only option; but the Imperial navies were based so far away (and so focused along trade routes around South Encyron) that naval invasions were not a big threat.
North Encyron is primarily temperate forest, with some alpine forests and arctic conditions towards the northernmost side; South Encyron is dominated by savannah and scrublands, with patches of tropical forest and desert.