Is it possible to log into my seed from one computer (light node) and at the same time log in from another computer or from android wallet?
Yes. Iota is a decentralized protocol. There is no central server managing any kind of session.
When you execute a transaction on one client, you do the pow to attach it to the tangle (it usually take a few seconds). Once the transaction is attached : the transaction is now a tip and is broadcasted on the network. The transaction will be visible in other client very soon (delay mainly depends of the network topology)