Since every transaction you conduct can have its own address:

If i receive 1.0 BTC  to address 1HSDRZYfcenZBNCZjk4y5wyxzPc79a6y33 and receive another 1.0 BTC to 12Ui8b9q6eqhTxZkow8H7VHbZafEnsPDs3 and then send 2.0 BTC
from address 2MJCaKXznp76qc69BwYDG4C28i8bNQng1N,  Can someone explain how the network is confirming there's sufficient funds for the transaction?

I'm sorry if this is a dumb question but I can't find any documentation that explains it.