Can you add more Dash to a masternode that already has 1000 Dash allocated for a masternode? If I had 2000 dash and made a master node with it. Do I get double the rewards vs a masternode with 1000 dash? Or do I have to separate the 2000 dash into 2 separate 1000 dash masternodes, hence running two nodes on the same computer or separate? How would one take advantage of this if they have more than 1000 dash?
You would have to create 2 masternodes and you would earn double the amount of Dash, and run separate nodes, probably on different computers as Dash requirements are sort of high.

Also you would need a server running 24/7 with 2 different IPs for the nodes.