I don't do Maryland tax returns, but that seems weird. I have never seen a state require 2 returns. From what I see in quick read of the instructions is you would file a RESIDENT return for MD and a part year for Delaware. You would prorate all income on MD to indicate income taxable to MD and do similar proration for DE. There may be a credit paid to another state for the duplication of income in MD while a DE resident. 

Maybe @sjrcpa has some experience