You should have started your own thread, because your question is not related to this threads subject.
The SPI and I2C (TWI) interfaces are buses and explicitly allow multiple sensors to connected concurrently. On the SPI bus every device gets it's own SS pin (shouldn't be a problem with a Mega) while on the I2C bus every device has to have a unique address in the range 0-127. Many chips provide (an) address pin(s) for that purpose to allow several chips of the same type to be connected.
For more information you need to specify what kind of sensors you want to connect and provide links to their datasheets.