(This is really an OpenVPN question, not a Tunnelblick question.)
There shouldn't be a problem if the computers are not connected to the VPN at the same time.
OpenVPN configurations usually include a security certificate that proves to the OpenVPN server that a client computer is legitimate.
An VPN can be set up to allow multiple connections using the same security certificate, or it can be set up to disconnect an existing connection when a new connection using the same security certificate connects.
It shouldn't do any harm to try to connect two computers to the VPN at the same time. Either it will work, or it will disconnect an existing connection on one computer when the other computer connects. If the first computer then connects again, it will disconnect the second one.