Adding two outlets mid circuit
Assuming that:


"outlet" refers to a typical dual-receptacle (because otherwise this wouldn't make sense - replace a single receptacle with a dual receptacle doesn't require putting in a bigger box)
Typical (i.e., not Chicago) NM (Romex) wiring


this is very straightforward:


Disconnect the existing receptacle
Run a new 12/2 cable from the original location to the new location
In the original location, connect the original black (hot) wire to two short black wires and the new 12/2 cable black wire
In the original location, connect the original white (neutral) wire to two short white wires and the new 12/2 cable white wire
In the original location, connect the original ground wire to two short bare wires and the new 12/2 cable ground wire
Connect each receptacle in the original location (one old, one new) to a set of hot/neutral/ground. Do NOT remove the tabs on the sides of the receptacles.
In the new location, connect hot/neutral/ground to the new receptacle