When I deploy my truffle smart contract on test networks like Ropsten and Kovan, can I interact with it using multiple PCs at once simultaneously? And will the data updated from one PC be visible on another?
Smart contracts are generally available to all users of the blockchain. Testnets like Ropsten and Kovan are public test blockchains which are available for everyone and therefore also all smart contracts in them are available for everyone.
One of the main points of blockchains is that multiple users can access them at the same time. So, yes, multiple users can use the same smart contract at the same time.