Mining Zcash with Nvidia 1080 TI + more?

Hello Zcashers,
bought 6 video cards 3 Zotac gtx 1080 TI and 3 Asus gtx 1080 TI and i want to build a mining rig. I want to mine Zcash but can i mine simultaneously Siacoins with harddrives is the first question? And if yes will i decrease my gpu mining profitability(hashes will go down ) doing it simultaneously or just Zcash mining will be the optimal solution?

Im fare new to it and appologies if i made some mistakes in terminology. Will appreciate all the info you can provide.

Thanks in advance

Assign 5 GPU to Zcash mining and one to Sia.