Hi Alejandro,
Let's call the 2 pcs PC1 and PC2.
If PC1 and PC2 are from different internet connections (even behind NAT), no problem.
If PC1 and PC2 are on the same network behind the same NAT router, then by default, they cannot both connect to the same PPTP server at the same time *unless* the NAT router they are using (be it a hardware NAT router or another SME server) has a PPTP client that can connect to the SME PPTP server at the other end. If it does, then both PC1 and PC2 can access the remote SME server at the same time.
IPSec is the way to go for LAN to LAN connections, if possible.
Kelvin