Let's say I build two transactions, and in the second one I add an input which refers to an output of the first one. Then I broadcast simultaneously the two transactions. Now, I know that in Bitcoin exists a Transaction Chaining mechanism, still there is a limit on how many transactions I can chain this way (25 as I remember). When that limit is reached, do I have to wait for the next block to chain more transactions, or just a couple of seconds so that each node can reconstruct the chain ? Thank you very much.
You need to wait for the transactions to be confirmed in a block. The limit is on the number of unconfirmed transactions that can be chained together and is completely unrelated to how quickly the transactions were received.
The chain can only be extended once the unconfirmed transaction chain shrinks, and that can only happen as transactions are confirmed. It is important to note that the entire chain may not be confirmed in the same block and that the next block may not include any transactions in the chain of unconfirmed transactions.