88E1111 Marvell PHY Chip Clock

There is no need to use both, unless you have more data than a gigabit Ethernet can handle. Note that Ethernet is full duplex, meaning each cable has dedicated TX and RX pins so there is no need to separate Tx and Rx into two cables. Trying to get both working at the same time is needlessly complex. Start with one of the examples that uses a single Ethernet and modify it to fit your needs. The Terasic software may not have been tested with both Ethernets in use at the same time. All I've seen are examples where there were separate projects to test each Ethernet.