Actually, the stamp duty on land without residential buildings kicks in at €10,001. As a stand-alone transaction a €20k parking space would probably incur stamp duty at 1%.
I don't believe the Revenue Commissioners would view the transactions as separate transactions as both transactions would probably be viewed as being dependant on the other. (I recall something about a set of assets rule or somehting like that in Stamp Duty law). I think in this case you would be stuck with paying the tax as if this was one transaction.