high density is single sided?

In general, yes.

then is low density ram and high density ram can be installed together in Emachines T6212 desktop?

my emachines have sticks of single sided and double sided and it reads all fine. so overall is this mean high density ram is compatible with Emachines T6212? and high and low desnsity ram can be installed together in Emachines T6212?
 
It depends on the motherboard. Although, unless you've already bought the RAM I wouldn't buy high density on the off chance that it will work.
 
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