only mountain in costa rica where you can see both atalntic and pacific oceans at the same time?

Irazu Volcano.

Climb to the heights of the Irazu Volcano and marvel over it's moon like surface. The Panaoramic view sometimes allows you to see the Pacific and Atlantic Oceans at the same time.

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An alternative is to go to Panama. There is a Condo Project were you can see the two Oceans from the Upper floors.
Of course bring money.

The only one mountain where you can see both oceans is the Cerro Chirripo an altitude of 3800mts +/-.. in summer you can see both. but in winter season is a little difficult to see them, so i recommend climb it in February is the best month for that amazing place...